diff --git a/crates/ui-egui/src/dialogs.rs b/crates/ui-egui/src/dialogs.rs
index 776ac82..52f9b41 100644
--- a/crates/ui-egui/src/dialogs.rs
+++ b/crates/ui-egui/src/dialogs.rs
@@ -581,7 +581,7 @@ impl Dialog {
                     },
                     Text { key: "errorMessage", label: "Error message:" },
                 ],
-                json!({"type": "list", "operator": "between", "ignoreBlank": true, "dropdown": true, "errorStyle": "stop"}),
+                validation_defaults(app),
             ),
             "cfQuick" => {
                 let ty = p("type").as_str().unwrap_or("cellIs").to_string();
@@ -813,6 +813,36 @@ impl Dialog {
     }
 }
 
+/// Initial values of the Data Validation form: the rule stored on the active cell, so OK without
+/// edits keeps it; a plain list rule when the cell has none.
+fn validation_defaults(app: &SheetApp) -> Json {
+    let mut v = json!({"type": "list", "operator": "between", "ignoreBlank": true, "dropdown": true, "errorStyle": "stop"});
+    let Some(doc) = app.session.active() else { return v };
+    let Some(dv) = doc.wb.active().and_then(|s| s.validations.iter().find(|d| d.ranges.iter().any(|r| r.contains(doc.selection.active)))) else {
+        return v;
+    };
+    // The enums serialize as their variant names; the command takes them in camelCase.
+    let camel = |x: Json| {
+        let name = x.as_str().unwrap_or_default();
+        let mut chars = name.chars();
+        chars.next().map(|c| c.to_lowercase().chain(chars).collect::<String>()).unwrap_or_default()
+    };
+    v["type"] = json!(camel(json!(dv.kind)));
+    v["operator"] = json!(camel(json!(dv.op)));
+    v["errorStyle"] = json!(camel(json!(dv.error_style)));
+    v["formula1"] = json!(dv.f1);
+    if let Some(f2) = &dv.f2 {
+        v["formula2"] = json!(f2);
+    }
+    v["ignoreBlank"] = json!(dv.allow_blank);
+    v["dropdown"] = json!(dv.in_cell_dropdown);
+    v["inputTitle"] = json!(dv.input_title);
+    v["inputMessage"] = json!(dv.input_message);
+    v["errorTitle"] = json!(dv.error_title);
+    v["errorMessage"] = json!(dv.error_message);
+    v
+}
+
 fn abs(r: &gridcraft_engine::core::RangeRef) -> String {
     let a = |c: gridcraft_engine::core::CellRef| format!("${}${}", gridcraft_engine::core::col_to_letters(c.col), c.row + 1);
     if r.is_single() { a(r.start) } else { format!("{}:{}", a(r.start), a(r.end)) }
@@ -2466,3 +2496,36 @@ fn spelling(app: &mut SheetApp, ui: &mut egui::Ui, d: &mut Dialog, open: &mut bo
         }
     });
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn data_validation_opens_on_the_rule_of_the_active_cell() {
+        let mut session = gridcraft_engine::Session::default();
+        session.new_workbook();
+        let mut app = SheetApp::new(session, Default::default());
+        app.run("data.validation", json!({"range": "A1", "type": "whole", "operator": "greater", "formula1": "5"})).unwrap();
+        let d = Dialog::open(&mut app, "dataValidation", json!({})).unwrap();
+        assert_eq!(d.values["type"], "whole");
+        assert_eq!(d.values["operator"], "greater");
+        assert_eq!(d.values["formula1"], "5");
+        // OK without edits keeps the rule: 6 is accepted, 4 is rejected.
+        app.run(d.command.unwrap(), build_params(&d)).unwrap();
+        let doc = app.session.active().unwrap();
+        let at = gridcraft_engine::core::CellRef::new(0, 0);
+        assert!(gridcraft_engine::cmd::data::check_validation(&doc.wb, 0, at, "6").is_none());
+        assert!(gridcraft_engine::cmd::data::check_validation(&doc.wb, 0, at, "4").is_some());
+    }
+
+    #[test]
+    fn data_validation_without_a_rule_opens_on_a_blank_list() {
+        let mut session = gridcraft_engine::Session::default();
+        session.new_workbook();
+        let mut app = SheetApp::new(session, Default::default());
+        let d = Dialog::open(&mut app, "dataValidation", json!({})).unwrap();
+        assert_eq!(d.values["type"], "list");
+        assert!(d.values.get("formula1").is_none());
+    }
+}