Scrolling issue

Hi,

   I have one user control and i put it in scroll viewer, and i have one drop down in the user control, so when i open the drop down then i scroll it up and down then drop down item is not scrolling, instead entire window is scrolling, can you tell me what could be a issue.

<ScrollViewer>

<User Control>

--I have drop down in the user control.

</User Control>

<ScrollViewer>