rjs gave you the best advice assuming that you have the information needed to comply. For a very long time brokerage houses were not required to keep track of your acquisition prices and dates, so they didn't. Your brokerage obviously doesn't have that information on your stock sale, but you should. The best way to avoid IRS scrutiny is to enter that information from your own records and indicate that it was not reported to the IRS by your brokerage. They didn't report it because they don't have it. Most likely this is a long term asset because brokerage houses are now required to keep that information on file and report it to the IRS when you make a sale. Without a declared cost basis for your asset, the IRS could impose taxes on the entire gross amount of your sale.
