Both columns are Column A, in different worksheets in the same workbook. They are both formatted as Text. I had this same issue when they were both formatted as General.

I have confirmed that the column headings in my list and my criteria match each other exactly.

Some of the matches seem to be matching on the first word only; in other words, 'WEST' is still being displayed in my List range, while what is in the criteria range has additional characters after that, starting with a space character.

The issue seems to be affecting only values where Column B in the worksheet where the List range is located, is blank. I have triple-checked, and I am not including Column B in the List range. This seems confirmed by the fact that, when the filter operation is finished running, only cells in Column A in the List range are selected.

I am comparing moderately large lists: about 35,000 rows in the List range and 1000 in the Criteria range. But even Excel 2007 should be able to handle this. I'm using Excel 2010. I paid for an upgrade specifically to try to get away from the various bugs in 2007, such as the "the selection is too complex to be copied" and other extremely frustrating errors. But at least some of those errors announce themselves with an explicit message...

I would sure appreciate any help. I can work around the problem for now, but I'm going to need to get this solved over the medium-term.