Cost basis Documentation - Mutual fund

[Michele-Frank]

I liquidated a mutual fund and there are 20+ years of dividend reinvestments incurred before the broker was required to  track cost basis.

I have all the statements and tabulated the cost basis.

What documentation does the IRS want to see for those years?
I can't believe they want me to end them all those statements!

[MaryK4]

For filing the 2025 return, you do not attach your spreadsheets or bank statements to your Form 1040.  You simply enter the total cost basis you calculated into TurboTax. The transaction will be reported on Form 8949 with Box E or Box F checked (indicating the basis was not reported to the IRS).

If the IRS ever questions the numbers (an audit), they will expect a contemporaneous record.  You should keep a file (digital or paper) containing a clear summary showing the date and amount of every reinvested dividend, the First and Last Statements to prove the initial purchase date and the final liquidation, and a few samples from the 20-year span to prove the dividend reinvestment plan was active.
 

[baldietax]

note if you enter this as a "sales summary" it will trigger a need to mail details to the IRS with Form 8453, your Form 8949 will say "see attached statement".  if you enter these sales instead as "one by one" with the details and any updated costs basis, this mailing need goes away.