If your only income is Social Security do you need to file tax returns?

Perhaps. It depends on whether you have enough non-Social Security income to be required to file an income tax return. Also, for 2007 and, possibly, 2008, you may qualify for an economic stimulus rebate, but only if you file a return. You can find a form on the internet or at the library. Just because IRS didn't send you a form, doesn't mean you are exempt from filing.