IE7 problem with drop down menu

I have built a dropdown menu on a site that works perfectly in IE8 and the usual Firefox, chrome and开发者_如何学JAVA safari.

However in IE7, the menu seems to sit behind the page when i use it? Has anyone come across something like this before? Thanks for reading.

Set a z-index of 1 on the nested ul assuming it's positioned relative or absolute and assuming there aren't any positioned siblings with z-indexes higher.