Yet more Excel help needed please

[DoubleCheese]

I've got two spreadsheets; one which lists of all our clients and their data on it and another that just lists some data but is just the currently open/active accounts. I'm not interested in the old and closed accounts off the first sheet.

I want to filter the first spreadsheet by having it compare and exclude everything but the active ones from the second sheet. Can anyone help with a formula that will look up the clients reference number and exclude the ones that aren't on the second list?

Thanks.