Whoa, Big Fella! What the original poster said was that his mother in law had no records. But There are records of the price history and the splits. So there IS a basis for a calculation. And the absolute LOWEST number you could come up with is the lowest price recorded over the period times the total number of shares sold (ignoring splits and reinvested dividends). That nunber, I will bet my hat, ass and overcoat, is greater than zero.

The IRS routinely accepts estimates to substantiate deduction/expense claims (eg business milage calculations,) as long as the result is reasonable. The fact that they sometimes (even if often) dispute such evidence is no reason not to try. So, I reiterate: “… never listen to someone who advises you to pass up a legitimate claim because you lack 'proof'".

As for the broker, if the IRS required, as it should, that brokerage firms require a basis for shares placed in their accounts, then that would settle that; and the brokerages, not to merntion the IRS and the tax practioners would be way ahead in man hours expended.