>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.
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If your rowsourcetype for the list is alias, sql statement, field or query and your list contains 253 or more elements then there is a bug that will get you eventually with all versions of VFP5 up to and including Service Pack 3. See MS KB Document Q164252 for a brief and not very thorough discussion of this problem (including a suggested workaround). In short, if your listindex mod 256 = 253 when the list gets the focus your program will be hosed. In your case since your listbox has 573 records then list items 253 and 509 will trigger the bug.

Murray