When you sell stock, you need toknow how much you paid for it. Anyprofit is taxable; any loss can be used tooffset profits. But finding that informationcan be hard. If you can't locate yourbrokerage statements, your brokershould be able to help. SEC rules requirebrokers to keep records for at least 6years and many keep them longer. If yourstock purchase is too far back to show upin your broker's records, you can estimatethe cost basis if you have some idea ofwhen you made the purchase. Athttp://finance.yahoo.com, you can getstock price data from past yearsâ€”justclick on "Stock Research" and, under"Research Tools," click on "HistoricalQuotes." If you estimate your costbasis, choosing the lowest price for thetime period may help keep the IRS at bay.