Thanks, Dan. I understand now.

The article at the MVPs site suggests using Null as the key value for the
UNION. This is the hidden column that shows the "(All"), so when you select
"(All)", Access recognises the combo value is Null, but Null doesn't match
anything, so it straightaway disconnects it from the "(All)" and just shows
a blank combo. Yes: I can see that happening.

A better solution would be to create another little table table with 2
fields:
   ID             Number    primary key
   TheText    Text    unique, and required.
Save as (say) tblAll4Combo.
Enter just one record, using -1 for the ID, and (All) for the text field.

Now try a RowSource of:
   SELECT CenterID,
   CenterName
   FROM tblCenterList
   UNION ALL
   Select ID AS CenterID,
   TheText AS CenterName
   FROM tblCenterList
   ORDER BY CenterID;
You should now be able to select the (All) option in the combo.

Now you will have the job of convincing the query to feed all values to the
report when -1 is chosen:

- If you are building a WhereCondition for OpenReport, your code will just
ignore the -1 value (i.e. don't filter on this field if the combo is -1.)

- If you are trying to make the query read the combo on your form, you will
need to modify the query so it returns all values when the combo is -1.
Switch the query to SQL View (View menu, in query design), locate the WHERE
clause, and change it so it looks like this:
   WHERE (([Forms].[Form1].[Combo2] = -1)
   OR ([Forms].[Form1].[Combo2] Is Null)
   OR (CenterID = [Forms].[Form1].[Combo2]))

I personally prefer the WhereCondition where this is possible: it's a bit
more work to program, but simpler and more efficient to execute.

Let us know how you go.