Queries :: Running Count Within Group

I have data like so:

Code:
SubjectID VisitID
1 5
1 5
2 7
2 7
2 9
2 9
etc

And need to return a running count for the number of visits per subject, so:

Code:
SubjectID VisitID VisitCount
1 5 1
1 5 1
2 7 1
2 7 1
2 9 2
2 9 2
etc.

I'd like to use DCount and have tried all variations of the following, but none of them returns the correct number:

VisitNumber: DCount("*","mytable","VisitID >= " & [VisitID] & " AND SubjectID =" & [SubjectID])