You need to show the stock sale, and if you do not know the cost basis you have a couple choices.
1) list it as $0 (which if small is OK, otherwise I do not recommend this)
2) try to reconstruct the price paid for the stock the best you can. For example 15 years ago the stock was listed for X dollars and my employee discount was 15%. Just use a sound logical method so you could show IRS (only if they ask) as to how the value was arrived at.
You can go the internet to research various sites for historical values and then may want to use a yearly average.
In TurboTax you can put in "Various" as purchase date, and then the program should ask if short or long term gain. Long term is anything over 1 year.
