Hi there,
The following could be assigned to a command button where the combo box on the the same form is called "cboCategories" (change to suit):
Select Case cboCategories.Value
Case Is = "A"
Rows(10 & ":" & 20).EntireRow.Hidden = False
Rows(21 & ":" & 50).EntireRow.Hidden = True
Case Is = "B"
Rows(20 & ":" & 30).EntireRow.Hidden = False
Rows(10 & ":" & 19).EntireRow.Hidden = True
Rows(31 & ":" & 50).EntireRow.Hidden = True
Case Is = "C"
Rows(30 & ":" & 40).EntireRow.Hidden = False
Rows(10 & ":" & 29).EntireRow.Hidden = True
Rows(41 & ":" & 50).EntireRow.Hidden = True
End Select
HTH
Robert