Hi, I'm using IDL 7.1 with windows 7 Home-Premium Edition 64-bits (on a Dell Studio 1747), and it seems nearly everytime I call the file_test function when it should return 0, I get the message "Fatar error : Due to a fatal error, the IDL session will now exit" with no further information on what happened.... Sometimes, my IDL workspace even gets "broken", as each time I reopen IDL inside it, I get the same error at the very start, when IDL is compiling my programs library... It gets really annoying, if anyone knows what could be the cause, or how to investigate the problem further, please let me know ! PS. It seems there is no link between files that cause this, I got the same errors from testing files on both of my hard drives. Also, I once got the same error from reading a specific ".fits.gz" file with either the readfits() and mrdfits() routines (from the astrolib library). However, this problem happened only with one or two .fits.gz files and not with the other ones.