In theory you should be able to do that by opening the Mail Merge Recipients
dialog box (I'm assuming you're using Word 2002/2003) and using the
drop-down in the label at the top of the column (let's assume the column is
called "Selected") to select only the rows where the value is "x" (be
careful about upper/lower case). Or use the Advanced... option in that
drop-down to go to a "Query options" dialog where you can specify the same
thing in a different way.

I say "in theory" because there can be problems with selections with some
data sources. But try that and see if it does what you need.

Peter Jamieson