diff --git a/src/os2/window.cpp b/src/os2/window.cpp
index 1f9123f388..c1f50464dc 100644
--- a/src/os2/window.cpp
+++ b/src/os2/window.cpp
@@ -755,6 +755,7 @@ void wxWindow::SetScrollbar(
                                  );
             }
             ::WinSendMsg(m_hWndScrollBarHorz, SBM_SETSCROLLBAR, (MPARAM)nPos, MPFROM2SHORT(0, (SHORT)nRange1));
+            ::WinSendMsg(m_hWndScrollBarHorz, SBM_SETTHUMBSIZE, MPFROM2SHORT((SHORT)nThumbVisible, (SHORT)nRange1), (MPARAM)0);
         }
     }
     else
@@ -804,6 +805,7 @@ void wxWindow::SetScrollbar(
                                  );
             }
             ::WinSendMsg(m_hWndScrollBarVert, SBM_SETSCROLLBAR, (MPARAM)nPos, MPFROM2SHORT(0, (SHORT)nRange1));
+            ::WinSendMsg(m_hWndScrollBarVert, SBM_SETTHUMBSIZE, MPFROM2SHORT((SHORT)nThumbVisible, (SHORT)nRange1), (MPARAM)0);
         }
         m_nYThumbSize = nThumbVisible;
     }