You have to report the sale on your tax return. Lacking any cost basis, the IRS will consider the entire sale amount as taxable. So, you need to make your best effort to determine the original cost basis, even if (worst case) it's a guess. Historical prices of most mutual funds are readily available on the internet and should, at least, satisfy the IRS that your basis wasn't zero; just "google" fund name (or symbol) Historical Prices
