diff --git a/src/queries.js b/src/queries.js
index 551456e..0d51707 100644
--- a/src/queries.js
+++ b/src/queries.js
@@ -80,6 +80,7 @@ export class ResultFilter {
     to_sql_where() { return "(1=1)"; }
     is_true() { return false; }
     is_false() { return false; }
+    is_negation() { return false; }
     simplify() { return this; }
 }
 
@@ -105,17 +106,23 @@ export class NegationFilter extends ResultFilter {
     body = new ConstFilter(this.return_type, false);
 
     to_sql_where() {
-        return "NOT (" + this.body.to_sql_where + ")";
+        return "NOT (" + this.body.to_sql_where() + ")";
     }
 
+    is_negation() { return true; }
+
     simplify() {
         var f = this.body.simplify();
+        console.log("NOT body:", f);
         if (f.is_true()) {
             return new ConstFilter(this.result_type, false);
         }
         if (f.is_false()) {
             return new ConstFilter(this.result_type, true);
         }
+        if (f.is_negation()) {
+            return f.body;
+        }
         return this;
     }
 }
diff --git a/src/userquerywidget.js b/src/userquerywidget.js
index 2ecfc50..cf18280 100644
--- a/src/userquerywidget.js
+++ b/src/userquerywidget.js
@@ -203,20 +203,29 @@ export function EditFilterExpressionDialog(props) {
         control = <EditMatchingFilterExpression {...subprops} />
     }
 
+    // If this is a "leaf" filter, we will allow changing the filter type in the dialog.
+    // But if it is a combination filter (i.e. AND / OR / NOT), we won't allow it because
+    // That throws away all its children.
+    const allowTypeChange =
+        (filter instanceof ConstFilter) ||
+        (filter instanceof MatchingFilter);
+
     return (
         <Dialog aria-labelledby={id} open={open}>
             <DialogTitle id={id}>Edit expression</DialogTitle>
-            <FormControl className={classes.margined}>
-                <Select
-                    labelId={selectLabelId}
-                    id={selectId}
-                    value={getFilterType(filter)}
-                    onChange={handleTypeChange}
-                >
-                    <MenuItem value={FilterTypeEnum.CONST}>Constant</MenuItem>
-                    <MenuItem value={FilterTypeEnum.MATCHING}>Matching</MenuItem>
-                </Select>
-            </FormControl>
+            {allowTypeChange &&
+                <FormControl className={classes.margined}>
+                    <Select
+                        labelId={selectLabelId}
+                        id={selectId}
+                        value={getFilterType(filter)}
+                        onChange={handleTypeChange}
+                    >
+                        <MenuItem value={FilterTypeEnum.CONST}>Constant</MenuItem>
+                        <MenuItem value={FilterTypeEnum.MATCHING}>Matching</MenuItem>
+                    </Select>
+                </FormControl>
+            }
             {control}
             <Button onClick={handleClose} className={classes.margined}>
                 Done
@@ -394,6 +403,40 @@ export function ConstFilterExpressionControl(props) {
     );
 }
 
+export function NegationExpressionControl(props) {
+    const classes = useStyles();
+    const { expr, onClick, onChange } = props;
+
+    var _ = require('lodash');
+    function handleBodyChanged(body) {
+        var new_filter = _.cloneDeep(expr);
+        new_filter.body = body;
+        onChange(new_filter);
+    }
+
+    return (
+        <>
+            <Box className={classes.logic_op_outer}>
+                <Box className={classes.logic_op_sbs}>
+                    <Box>
+                        <Box className={classes.logic_op_subexpr}>
+                            <FilterExpressionControl expr={expr.body} onChange={handleBodyChanged} isRoot={false} />
+                        </Box>
+                    </Box>
+                </Box>
+                <Button
+                    variant="outlined"
+                    className={classes.filterexpcontrol + " " + classes.logic_op_sbs}
+                    aria-controls="simple-menu" aria-haspopup="true"
+                    onClick={onClick}
+                >
+                    NOT
+                </Button>
+            </Box>
+        </>
+    )
+}
+
 export function FilterExpressionControl(props) {
     const { expr, onChange, isRoot } = props;
     const [anchorEl, setAnchorEl] = React.useState(null);
@@ -428,6 +471,12 @@ export function FilterExpressionControl(props) {
     };
 
     const handleRemove = () => {
+        // For negation filters, removal means replacing the negation node by its child.
+        // In all other cases, removal means complete deletion of the subtree.
+        if (expr instanceof NegationFilter) {
+            onChange(expr.body);
+            return;
+        }
         onChange(null);
     }
 
@@ -458,6 +507,12 @@ export function FilterExpressionControl(props) {
         setCombineDialogOpen(true);
     }
 
+    const handleNegation = () => {
+        handleCloseMenu();
+        var new_filter = new NegationFilter(expr.result_type, expr);
+        onChange(new_filter.simplify());
+    }
+
     var filter_elem = false;
     if (expr instanceof ConstFilter) {
         filter_elem = <ConstFilterExpressionControl {...props} onClick={handleClick} />
@@ -465,10 +520,17 @@ export function FilterExpressionControl(props) {
         filter_elem = <LogicalOperatorFilterExpressionControl {...props} onClick={handleClick} onChange={onChange} />
     } else if (expr instanceof MatchingFilter) {
         filter_elem = <MatchingFilterExpressionControl {...props} onClick={handleClick} />
+    } else if (expr instanceof NegationFilter) {
+        filter_elem = <NegationExpressionControl {...props} onClick={handleClick} onChange={onChange} />
     } else {
         throw new Error('Unsupported filter expression');
     }
 
+    // If this is the root node, removing it is not allowed.
+    // Other nodes may be removed.
+    // The only exception is a negation node: removing that will replace it by its child.
+    var allowRemove = !isRoot || (expr instanceof NegationFilter);
+
     return (
         <>
             {filter_elem}
@@ -482,7 +544,8 @@ export function FilterExpressionControl(props) {
                 <MenuItem onClick={handleOpenEditDialog}>Edit...</MenuItem>
                 <MenuItem onClick={handleAnd}>And...</MenuItem>
                 <MenuItem onClick={handleOr}>Or...</MenuItem>
-                {!isRoot && <MenuItem onClick={handleRemove}>Remove</MenuItem>}
+                <MenuItem onClick={handleNegation}>Negate</MenuItem>
+                {allowRemove && <MenuItem onClick={handleRemove}>Remove</MenuItem>}
             </Menu>
             <EditFilterExpressionDialog
                 onClose={handleCloseEditFilterDialog}