diff --git a/crates/forms/src/appearance.rs b/crates/forms/src/appearance.rs
index e5fe07eb9..23d00e21f 100644
--- a/crates/forms/src/appearance.rs
+++ b/crates/forms/src/appearance.rs
@@ -358,8 +358,8 @@ fn is_round(doc: &Document, wd: &Dict) -> bool {
 
 /// [`frame`] for a round check button: the `/MK /BG` background as a disc and the `/MK /BC`
 /// border as a circle, both centred in the `w` × `h` box and as wide as its shorter side. A
-/// dashed border stays dashed; any other style is drawn solid, and a border as wide as the
-/// circle fills it. Returns the border width.
+/// dashed border stays dashed and an underline stays an underline; any other style is drawn
+/// solid, and a border as wide as the circle fills it. Returns the border width.
 fn round_frame(doc: &Document, wd: &Dict, mk: Option<&Dict>, w: f64, h: f64) -> (String, f64) {
     let mut c = String::new();
     let (cx, cy, r) = (w / 2.0, h / 2.0, w.min(h).max(0.0) / 2.0);
@@ -368,14 +368,20 @@ fn round_frame(doc: &Document, wd: &Dict, mk: Option<&Dict>, w: f64, h: f64) ->
     }
     let bs = wd.get(b"BS").map(|b| doc.resolve(b)).and_then(|b| b.as_dict().cloned());
     let bw = bs.as_ref().and_then(|b| b.get(b"W")).and_then(|x| doc.resolve(x).as_f64()).unwrap_or(1.0).max(0.0);
+    let style = bs.as_ref().and_then(|b| b.name(b"S").map(<[u8]>::to_vec)).unwrap_or_default();
     match color_array(doc, mk, b"BC") {
+        // An underline stays one: only the bottom edge, as for any widget.
+        Some(bc) if bw > 0.0 && style == b"U" => {
+            c.push_str(&format!("{}\n{} w\n0 {} m {} {} l S\n", stroke_op(&bc), n(bw), n(bw / 2.0), n(w), n(bw / 2.0)));
+            (c, bw)
+        }
         // A border as wide as the circle fills it: a disc in the border's colour.
         Some(bc) if bw > 0.0 && bw >= 2.0 * r => {
             c.push_str(&format!("{bc}\n{}f\n", circle_path(cx, cy, r)));
             (c, 2.0 * r)
         }
         Some(bc) if bw > 0.0 => {
-            let dashed = bs.as_ref().and_then(|b| b.name(b"S")) == Some(b"D");
+            let dashed = style == b"D";
             // The stroke is centred on the path: inset by half its width, it stays inside.
             let path = circle_path(cx, cy, r - bw / 2.0);
             c.push_str(&format!("{}\n{} w\n{}{path}S\n[] 0 d\n", stroke_op(&bc), n(bw), if dashed { "[3] 0 d\n" } else { "" }));
diff --git a/crates/forms/src/tests.rs b/crates/forms/src/tests.rs
index 4255cbac8..8aa34f0e1 100644
--- a/crates/forms/src/tests.rs
+++ b/crates/forms/src/tests.rs
@@ -389,7 +389,8 @@ fn opt_export_values_select_button_states() {
 
 /// A check box laid out for a round XFA check button (`/PCXfaRound true`) draws its background
 /// and border as a circle inside the widget, inset by half the border so the stroke stays in,
-/// or a disc when the border is as wide as the circle; one without the key keeps its rectangle.
+/// or a disc when the border is as wide as the circle (an underline stays one); one without the
+/// key keeps its rectangle.
 #[test]
 fn a_round_xfa_check_button_draws_a_round_background_and_border() {
     let widget = |name: &str, extra: &str| {
@@ -400,15 +401,16 @@ fn a_round_xfa_check_button_draws_a_round_background_and_border() {
     let objs: Vec<String> = vec![
         "<< /Type /Catalog /Pages 2 0 R /AcroForm 4 0 R >>".into(),
         "<< /Type /Pages /Kids [3 0 R] /Count 1 /MediaBox [0 0 600 800] >>".into(),
-        "<< /Type /Page /Parent 2 0 R /Annots [5 0 R 6 0 R 7 0 R 8 0 R] >>".into(),
-        "<< /Fields [5 0 R 6 0 R 7 0 R 8 0 R] >>".into(),
+        "<< /Type /Page /Parent 2 0 R /Annots [5 0 R 6 0 R 7 0 R 8 0 R 9 0 R] >>".into(),
+        "<< /Fields [5 0 R 6 0 R 7 0 R 8 0 R 9 0 R] >>".into(),
         widget("round", " /PCXfaRound true"),
         widget("square", ""),
         widget("dashed", " /PCXfaRound true /BS << /W 2 /S /D >>").replace("/BS << /W 1 /S /S >>", ""),
         widget("thick", " /PCXfaRound true /BS << /W 30 /S /S >>").replace("/BS << /W 1 /S /S >>", ""),
+        widget("underlined", " /PCXfaRound true /BS << /W 1 /S /U >>").replace("/BS << /W 1 /S /S >>", ""),
     ];
     let mut doc = document(&objs);
-    for name in ["round", "square", "dashed", "thick"] {
+    for name in ["round", "square", "dashed", "thick", "underlined"] {
         redraw_field(&mut doc, name).unwrap();
     }
     // The Off state: only the frame.
@@ -432,6 +434,9 @@ fn a_round_xfa_check_button_draws_a_round_background_and_border() {
     // A border wider than the 20pt circle fills it: a disc in the border's colour, no stroke.
     let thick = off("thick");
     assert!(thick.contains("1 0 0 rg\n20 10 m") && !thick.contains(" S\n"), "a border as wide as the circle is a disc: {thick}");
+    // An underline stays the bottom edge, under a round background.
+    let underlined = off("underlined");
+    assert!(underlined.contains("1 0 0 RG\n1 w\n0 0.5 m 20 0.5 l S"), "an underline stays an underline: {underlined}");
 }
 
 /// Issue #1323: a text field without a format keeps a whitespace-only value as typed; only an