diff --git a/comctl_windows.go b/comctl_windows.go
index ddcad10..1c3014f 100644
--- a/comctl_windows.go
+++ b/comctl_windows.go
@@ -10,6 +10,8 @@ import (
 // pretty much every constant here except _WM_USER is from commctrl.h
 // TODO for all: filter out constants not available in Windows XP
 
+// TODO ensure that comctl32.dll version 6 or newer is loaded as it provides marquee progress bars
+
 // InitCommonControlsEx constants.
 const (
 	_ICC_LISTVIEW_CLASSES = 0x00000001
@@ -67,6 +69,7 @@ const (
 const (
 	_PBS_SMOOTH = 0x01
 	_PBS_VERTICAL = 0x04
+	_PBS_MARQUEE = 0x08
 )
 
 // Progress Bar messages.
@@ -81,4 +84,5 @@ const (
 	_PBM_GETPOS = (_WM_USER + 8)
 	_PBM_SETBARCOLOR = (_WM_USER + 9)
 	_PBM_SETBKCOLOR = _CCM_SETBKCOLOR
+	_PBM_SETMARQUEE = (_WM_USER + 10)
 )
diff --git a/sysdata_windows.go b/sysdata_windows.go
index fbba1fd..5ef9209 100644
--- a/sysdata_windows.go
+++ b/sysdata_windows.go
@@ -15,6 +15,7 @@ type sysData struct {
 	children			map[_HMENU]*sysData
 	nextChildID		_HMENU
 	childrenLock		sync.Mutex
+	isMarquee		bool			// for sysData.setProgress()
 }
 
 type classData struct {
@@ -514,9 +515,71 @@ func (s *sysData) delete(index int) {
 	}
 }
 
-func (s *sysData) setProgress(percent int) {
+func (s *sysData) setIndeterminate() {
 	ret := make(chan uiret)
 	defer close(ret)
+	uitask <- &uimsg{
+		call:		_setWindowLong,
+		p:		[]uintptr{
+			uintptr(s.hwnd),
+			uintptr(_GWL_STYLE),
+			uintptr(classTypes[s.ctype].style | _PBS_MARQUEE),
+		},
+		ret:		ret,
+	}
+	r := <-ret
+	if r.ret == 0 {
+		panic(fmt.Errorf("error setting progress bar style to enter indeterminate mode: %v", r.err))
+	}
+	uitask <- &uimsg{
+		call:		_sendMessage,
+		p:		[]uintptr{
+			uintptr(s.hwnd),
+			uintptr(_PBM_SETMARQUEE),
+			uintptr(_WPARAM(_TRUE)),
+			uintptr(0),
+		},
+		ret:		ret,
+	}
+	<-ret
+	s.isMarquee = true
+}
+
+func (s *sysData) setProgress(percent int) {
+	if percent == -1 {
+		s.setIndeterminate()
+		return
+	}
+	ret := make(chan uiret)
+	defer close(ret)
+	if s.isMarquee {
+		// turn off marquee before switching back
+		uitask <- &uimsg{
+			call:		_sendMessage,
+			p:		[]uintptr{
+				uintptr(s.hwnd),
+				uintptr(_PBM_SETMARQUEE),
+				uintptr(_WPARAM(_FALSE)),
+				uintptr(0),
+			},
+			ret:		ret,
+		}
+		<-ret
+		uitask <- &uimsg{
+			call:		_setWindowLong,
+			p:		[]uintptr{
+				uintptr(s.hwnd),
+				uintptr(_GWL_STYLE),
+				uintptr(classTypes[s.ctype].style),
+			},
+			ret:		ret,
+		}
+		r := <-ret
+		if r.ret == 0 {
+			panic(fmt.Errorf("error setting progress bar style to leave indeterminate mode (percent %d): %v", percent, r.err))
+		}
+		s.isMarquee = false
+	}
 	uitask <- &uimsg{
 		call:		_sendMessage,
 		p:		[]uintptr{
diff --git a/todo.md b/todo.md
index a434321..c572b65 100644
--- a/todo.md
+++ b/todo.md
@@ -74,6 +74,7 @@ important things:
 - make passing of parameters and type conversions of parameters to uitask on Windows consistent: explicit _WPARAM(xxx)/_LPARAM(xxx)/uintptr(xxx), for example
 	- do this for type signatures in exported functions: (err error) or just error?
 	- do this for the names of GTK+ helper functions (gtkXXX or gXXX)
+- on windows 7, progress bars seem to animate from 0 -> pos when you turn off marquee mode and set pos; see if that's documented or if I'm doing something wrong
 
 far off:
 - localization
diff --git a/windows_windows.go b/windows_windows.go
index 9bdf52c..9ed12f9 100644
--- a/windows_windows.go
+++ b/windows_windows.go
@@ -105,8 +105,8 @@ const (
 	_COLOR_INFOBK = 24
 	_COLOR_INFOTEXT = 23
 	_COLOR_MENU = 4
-	COLOR_MENUHILIGHT = 29
-	COLOR_MENUBAR = 30
+	_COLOR_MENUHILIGHT = 29
+	_COLOR_MENUBAR = 30
 	_COLOR_MENUTEXT = 7
 	_COLOR_SCROLLBAR = 0
 	_COLOR_WINDOW = 5
@@ -168,6 +168,7 @@ var (
 	_createWindowEx = user32.NewProc("CreateWindowExW")
 	_getClientRect = user32.NewProc("GetClientRect")
 	_moveWindow = user32.NewProc("MoveWindow")
+	_setWindowLong = user32.NewProc("SetWindowLongW")
 	_setWindowPos = user32.NewProc("SetWindowPos")
 	_setWindowText = user32.NewProc("SetWindowTextW")
 	_showWindow = user32.NewProc("ShowWindow")
@@ -241,7 +242,7 @@ const (
 )
 
 // WM_STYLECHANGED and WM_STYLECHANGING values (wParam).
-const (
+var (		// var because Go won't let me cast a negative const to a uintptr
 	_GWL_EXSTYLE = -20
 	_GWL_STYLE = -16
 )