>>
>>thanks, but no, clientType is never null.
>
>Assuming clientType isn't null, something else in that code is failing (probably because of a null).

Thanks again (and Mike too).

In the meantime I had changed the code to this:

        private void SetClientTypeDisplay(string clientType)
        {
            this.pnlCompany.Visible = (clientType.Trim() == "Company");

and that works, so I guess I'll never work out what the problem was (unless I get a chance to put it back to the original and step through it).