The basis of the stock you sold is the cost of the shares plus any adjustments, such as sales commissions. If you have not kept detailed records of your dividend reinvestments, you may be able to reconstruct those records with the help of public records from sources such as the media, your broker, or the company that issued the dividends.

If you cannot specifically identify which shares were sold, you must use the first-in first-out rule. This means that you deem that you sold the oldest shares first, then the next oldest, until you have accounted for the number of shares in the sale. In order to establish the basis of these shares, you need to have kept adequate documentation of all your purchases, including those that were made through the dividend reinvestment plan. You may not use an average cost basis. Only mutual fund shares may have an average cost basis.

Refer to Publication 550, Investment Income and Expenses, and Publication 551, Basis of Assets.

References:

• Publication 550, Investment Income and Expenses
• Publication 551, Basis of Assets
• Tax Topic 404, Dividends

Source: www.irs.gov