EDIT I didnt realize this question was for a report and not a form until after I posted so this will not apply, sorry!
This will do it (Code between the bold text):
Private Sub txtYourTextbox_Exit(Cancel As Integer)
If IsNull(Me.txtYourTextbox) Then
lblYourLabel.Visible = False
End If
End Sub
[This message has been edited by Talismanic (edited 01-12-2001).]