The built in user manual says the following: ========================================== MzScheme supports multiple threads of control within a program. Threads are implemented for all operating systems, even when the operating system does not provide primitive thread support. (thread thunk) invokes the procedure thunk with no arguments in a new thread of control. The thread procedure returns immediately with a thread descriptor value. When the invocation of thunk returns, the thread created to invoke thunk terminates. Example: (thread (lambda () (sleep 2) (display 7) (newline))) ; => a thread descriptor  displays 7 after two seconds pass ========================================== > (thread (lambda () (sleep 2) (display 7) (newline))) #<thread:932:10> hash-table-get: expects type <hash-table> as 1st argument, given: #f; other arguments were: profile-point20003 i then tried doing it in a "clean" environment, (i.e. a environment in which none of my programs had been run) and it worked fine.  Wthat could I be doing that could be causing this? The following are things that I could imagine might be related: * My program uses a GUI (and therefore threads are already happening) * My version is a bit old:      Welcome to DrScheme, version 299.406-svn25nov2005. * I have profiling switched on:      Language: Pretty Big (includes MrEd and Advanced Student) custom... ***Ok, I just tried it with profiling turned off, and things worked fine.  Is there a conflict between profiling and threads? Corey -- ((lambda (y) (y y)) (lambda (y) (y y)))