Why it matters

Cost basis determines the capital gain or loss you report when you sell an investment. An incorrect basis can cause you to overpay taxes or face an IRS adjustment. Brokers often report basis on Form 1099‑B, but mistakes happen—especially after transfers, stock splits, DRIPs, or corporate actions. (See IRS guidance: Form 8949 and Schedule D, and Publication 551 for basis rules.)

Quick step-by-step fix

  1. Verify the error
  • Gather trade confirmations, account statements, transfer records, and any corporate action notices (splits, mergers, spin‑offs). Confirm purchase date, number of shares, and total cost (including commissions).
  1. Ask the broker to investigate and correct
  • Contact your brokerage’s cost‑basis or tax reporting team. Ask them to recheck internal records and, if they agree, issue a corrected Form 1099‑B (a “Corrected 1099‑B”). Keep all written correspondence and ticket numbers.
  • Typical result: if the broker corrects the 1099‑B before you file, use the corrected form to prepare Form 8949 and Schedule D.
  1. Report the correct basis on your return
  • If the broker fixes the 1099‑B before filing, report the corrected figures on Form 8949 and Schedule D when you file your tax return (IRS, Form 8949; IRS, Schedule D).
  • If the broker won’t correct or you discover the mistake after filing, report the correct basis on Form 8949 and Schedule D and, if you already filed, file Form 1040‑X to amend your return (IRS, Form 1040‑X).
  1. File an amended return when necessary
  • Use Form 1040‑X to claim a refund or fix tax due when the basis error affected your reported gain or loss. Attach a corrected Form 8949, Schedule D, and any broker statements showing the corrected basis.
  • Note timing: generally you have three years from your original filing date to claim a refund, but talk with a tax pro about exceptions. (IRS, About Form 1040‑X.)

Documentation checklist

  • Trade confirmations for purchases and sales
  • Account statements showing transfers and reinvested dividends (DRIPs)
  • Broker 1099‑B(s) and any corrected 1099‑B
  • Notices of corporate actions (splits, consolidations, mergers)
  • Written correspondence with the broker and any cost‑basis worksheets you maintain

Real-world examples (short)

  • Split not applied: After a 2‑for‑1 split, a client’s broker reported the pre‑split per‑share price, doubling the reported gain. The broker issued a corrected 1099‑B after we provided the split notice and original trade confirmations.
  • Transfer mismatch: When securities move between custodians, many lots can be lost or relabeled; reconstructing trades from old statements usually resolved the discrepancy.

When to contact a tax pro

  • Multiple-year cost basis reconstructions
  • Complex corporate actions or inherited/gifted property basis calculations
  • When an IRS notice arrives claiming unreported income or mismatched 1099‑B data

Common pitfalls

  • Assuming the broker automatically fixed the basis—always get a written corrected 1099‑B.
  • Using calendar year statements without matching trade settlement dates (look at settlement, not just trade date).
  • Forgetting adjustments for wash‑sale rules or return of capital events—these affect basis and timing.

Record retention and timing

Keep records that establish basis at least until three years after you file the return reporting the sale—many advisors keep investment basis records indefinitely. If you expect to amend prior years, gather records going back to the original purchase dates.

Authority and where to learn more

Internal resources

Professional note

In my 15 years advising investors, the fastest fixes come from good recordkeeping and an immediate, documented request to the broker. If you’re facing IRS notices or multi‑year reconstructions, engage a CPA or enrolled agent to avoid missed adjustments.

Disclaimer

This article is educational and not a substitute for personalized tax advice. For decisions about filing or amending returns, consult a qualified tax professional.