Thanks for the prompt response.
I already coded for your first option, but I thought it would be nicer to actually disable the button until the required data was completed.
Without boring you too much with the details...
The sheet has a drop-down box allowing the user to select a specific record from a data sheet and enter some other data against this, resulting in a number of calculated values being added to the original record.
So what I want is to disable the button until an item is selected.
Please don't spend ages on this, as I can just leave it with an error message as per the current solution. I just wondered whether the disabling of a button is possible in this scenario.
It is possible, it just depends on whether the validation of your requirements is based on a calculation or a user changing a cell.
Try something like this in the Worksheet code module:
Private Sub CommandButton1_Click()
Private Sub Worksheet_Change(ByVal Target As Range)
If Range("B1") = "True" Then
Me.CommandButton1.Enabled = True
Me.CommandButton1.Enabled = False