cost basis unknown

[cher52]

I have no historical records for the deceased taxpayers sale of a stock...the only records I have are 199-div dating back to 2017......it is possible that the stock was bought as far back as 2008 or earlier....is it unlikely that the irs would go back beyond my 2017 1099-div to find another cost basis......

[ThomasM125]

It is not likely they would take the time and effort to determine the original cost basis, but that would not prevent them from disallowing your estimated cost basis if you didn't have evidence to support it. It is up to you to substantiate the entries on your tax return, so they could deny your cost basis and reduce it to $0 if you couldn't provide evidence that it was accurate.