How should I report the cost basis of sold stocks if their original purchase date and price is unknown?

[CarolynM · Accepted solution]

You should use your best approximation of the purchase date and the historical stock price to report the cost basis. 

If the valuation of the stock was unlikely to be very different on June 1, 2010 than June 15, 2010.

If you are working in good faith, the IRS cannot fault you for your approximation efforts.

Follow this link for a tool for historical stock prices: Historical Stock Prices