Thank you Bob, I got someone to change your code so the operator can only
chose one of the three options. Here is the code should it help someone else
as you can see it runs with no problem with large amounts of data.

Regards

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count 1 Then Exit Sub
If Not Intersect(Target, Range("t2:t25036")) Is Nothing Then
Target.Font.Name = "Marlett"
If Target = vbNullString Then
Target = "a"
Target.Offset(0, 1) = vbNullString
Target.Offset(0, 2) = vbNullString
Else
Target = vbNullString
End If
ElseIf Not Intersect(Target, Range("u2:u25036")) Is Nothing Then
Target.Font.Name = "Marlett"
If Target = vbNullString Then
Target = "a"
Target.Offset(0, -1) = vbNullString
Target.Offset(0, 1) = vbNullString
Else
Target = vbNullString
End If
ElseIf Not Intersect(Target, Range("v2:v25036")) Is Nothing Then
Target.Font.Name = "Marlett"
If Target = vbNullString Then
Target = "a"
Target.Offset(0, -2) = vbNullString
Target.Offset(0, -1) = vbNullString
Else
Target = vbNullString
End If
End If
End Sub

"Miss Marple" wrote:

I have a large spreadsheet containing 1000s of rows which an operator will
have to consider and against each row mark Yes, No or Maybe - with a single
mouse click. I have tried using tick boxes but cannot copy the control down
to be able to manipulate the data at a later date. What would be the best
method?

Thank you