With content control dropdowns/comboboxes you can only select one item per dropdown/combobox. For an ActiveX listbox to work, you'd need to have a macro populate that listbox when the document is created/opened and, if you make selections, store those so the macro can re-select them next time the document is opened. That, in turn, means either storing the code that does the work in the document's template (meaning the document will only work on PCs that can access the template over your network or, if you use stand-alone PCs, the document will only work on PCs that the template has been installed on) or you'll have to save the document in the docm format - which is really only suitable if you're re-using the same document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]