Remove rows in excel if not in list

Hello,

I have an excel spreadsheet with chart number and data, many chartnumbers are there multiple times (which is correct). 36K rows. I want to remove patients who aren't in another list (a spreadsheet of those patients included in my study, a list of some 400 patients, one record for each person).

How do I remove the rows from the first file, which aren't in the second list. Or vice versa, select those rows that are in the second list.

Thanks,
Neil