Missing Cost Basis on Form 1099-B 2026 – Reporting Considerations for 2025 Investment Sales

Missing cost basis on Form 1099-B generally indicates a gap in broker reporting or supporting information, rather than establishing that an investment had no acquisition cost. Because gain or loss depends on adjusted basis, an unresolved basis figure may materially affect the reported result of a stock sale.

This publication explains the distinction between sale proceeds, basis supplied by a broker, and basis reported to the IRS. It addresses 2025 investment sales reported during the 2026 filing season and includes California reporting considerations. The discussion concerns stocks and conventional brokerage securities, rather than digital-asset reporting. It is educational and general; no specific taxpayer facts or circumstances are evaluated.

1. Missing Basis and Noncovered Securities

Form 1099-B may be delivered as part of a consolidated brokerage tax statement. The package can contain separate transaction groups based on holding period and whether basis was reported to the IRS.

For certain noncovered securities, the broker is permitted to leave acquisition-date and basis fields blank. A noncovered designation concerns the broker's reporting obligations; it does not establish that the sale is exempt from tax.

A blank field is not itself evidence of zero basis. Conversely, the presence of a number does not establish that every required adjustment has been reflected. These distinctions are relevant to the Form 8949 reporting framework.

2. Sale Proceeds Compared with Adjusted Basis

For a straightforward sale of purchased stock, gain or loss generally reflects the difference between the amount realized and adjusted basis. Purchased-property basis typically begins with acquisition cost; applicable costs and later adjustments can affect the calculation.

The following hypothetical example assumes that proceeds already reflect selling expenses, adjusted basis is supported by records, and no further adjustments apply.

Using zero in place of the supported basis would produce a $25,000 gain in this simplified calculation. The difference illustrates the effect of a reporting error, rather than a promised tax reduction. The example does not establish basis for an actual sale. Different treatment may apply to inherited property, gifts, employee-related shares, or transactions requiring other adjustments.

3. Records Commonly Associated with Basis Reconciliation

Records associated with a basis review commonly include purchase confirmations, historical account statements, reinvestment histories, transfer documentation, and notices of corporate actions. Their usefulness depends on whether they can be connected to the particular shares sold.

A statement showing the total value of an account may provide less information than transaction-level records. Similarly, a current account balance does not establish the acquisition cost of a historical sale.

When shares moved between brokers, the history held by the original institution may help explain information missing from the receiving institution's tax package. Reconciliation can involve the quantity transferred, acquisition information, and the particular shares later sold.

For inherited or gifted investments, ordinary purchase-cost assumptions may not apply. IRS Publication 551 describes inheritance and gift basis rules, including circumstances in which the basis used to calculate a gain differs from the basis used to calculate a loss. The value shown on the date of a transfer is not necessarily the applicable tax basis.

The documentation question is therefore more specific than whether an investor has a brokerage statement. It concerns whether the available evidence supports the basis and holding-period treatment of the reported disposition.

4. Form 8949 Reporting Categories and Adjustments

Form 8949 generally reconciles capital-asset transactions with information reported on Forms 1099-B and other applicable statements. The resulting totals are generally carried to Schedule D, subject to the relevant reporting exceptions.

The reporting approach depends partly on whether basis was reported to the IRS. An incorrect reported figure and an unreported basis figure can require different treatment. The instructions describe the relevant categories and adjustment procedures.

Holding-period classification is also relevant. Capital gains and losses are generally short-term when an asset was held one year or less and long-term when held more than one year, subject to exceptions. Acquisition information can therefore affect classification as well as the calculation itself.

5. California Capital-Gain Treatment

California does not provide preferential capital-gain tax rates. Capital gains are taxed at ordinary income rates for California personal income-tax purposes. The FTB identifies California Schedule D (540) as applicable when federal and California capital-gain treatment differs.

Basis accuracy is consequently relevant to both returns. Where federal and California adjustments differ, separate calculations may be necessary. A federal result does not, by itself, demonstrate that the state result is identical.

The broader filing framework is discussed in Tax Extensions 2026 and the October 15 individual filing checklist. The separate payment framework for current-year investment income is discussed in Quarterly Estimated Taxes 2026. Those articles address related issues beyond this publication's basis-reporting scope.

Compliance Resources and Tools

• Internal Revenue Service — 2025 Instructions for Form 1099-B, Proceeds From Broker and Barter Exchange Transactions (accessed October 5, 2026).
• Internal Revenue Service — Instructions for Form 8949 (2025), Sales and Other Dispositions of Capital Assets (accessed October 5, 2026).
• Internal Revenue Service — Topic No. 409, Capital Gains and Losses (accessed October 5, 2026).
• Internal Revenue Service — Publication 551 (12/2025), Basis of Assets (accessed October 5, 2026).
• California Franchise Tax Board — Capital Gains and Losses (accessed October 5, 2026).