Barbara...
Thanks for the posting... I found out what the deal is through the VFP Knowledgebase web. VFP 5.0 has a bug here. The only way to keep the selected items is to set the RowSourceType to 0 (None) and poplulate through the AddItem() function. mmm... They never make it easy.

>I have a VFP5 listbox (original form, not subclassed)
>
>RowSourceType: Alias
>RowSource: PLATE.DBF
>Multiselect: TRUE
>
>The listbox works perfectly when it first gets focus, including selecting with SHIFT or CTRL keys. However, if I focus on another control and come back to the listbox, all of the previously selected items are unselected even if I have the CTRL key presssed. I realize that focussing on another control and then clicking on the list box WITHOUT the CTRL key pressed will un-select all items except the one clicked on. This problem is different.
>
>This is NOT "standard windows" and it is not the way VFP3 handled the situation. In VFP3 I had 4 list boxes and users could move back and forth selecting/unselecting values in each.
>
>Advice? Work-around?
>
>TIA
>Barbara