what to do if debug runs fine, but release crashes

I have an application that runs just fine in the debug build, but when I start it in the release build, I get a

unhandled Exception at 0x0043b134 in myapp.exe: 0xC0000005:
Access violation while reading at position 0x004bd96c

If I click on 'break' it tells me that there are no symbols loaded and the source code can't be displayed.

What can I do in such a situation to track down the problem?