Thanks for your help John, but the problem wasn't related to memory at all. I deleted the listbox controls (who were instanciated from my base listbox class), used the VFP listbox instead and it work very well now.

It seem that my listbox class got corrupted.

>OK. What is your SET RESOURCE setting? If it's ON and you don't specifically need the FOXUSER table, set it OFF (will conserve some memory). Secondly, there are two SYS() functions that show GDI (object) memory and allow you to set it...I cannot find those functions in my VFP Help file but I recall them being documented in "The Hacker's Guide to Visual FoxPro" and maybe some other UT member can help with that?