What If My Cost Basis Is Missing or Wrong?

The direct answer

If your broker shows no basis—or shows a basis you believe is wrong—the tax answer is not automatically zero and it is not automatically whatever appears on the brokerage statement.

You need to determine the legally correct adjusted basis from the way the property was acquired and the events that changed its basis afterward.

A blank Form 1099-B can be entirely legitimate. In particular, noncovered securities may be reported without basis under the broker-reporting rules. That does not mean the investment actually has no tax basis.

Missing broker basis and zero tax basis are different things

This distinction is fundamental.

Broker basis reporting is an information-reporting system. Your substantive tax basis is determined under federal tax law.

Those systems overlap, but they are not identical.

A broker may lack information about events that occurred before an asset reached the account or outside the broker's reporting obligations. Transfers between institutions, old purchases, gifts, inheritances, employer stock, wash sales, and corporate actions can all create histories that a current brokerage screen does not fully capture.

So:

“Basis not reported” does not mean “basis equals zero.”

At the same time, missing records do not give a taxpayer permission to invent a convenient number.

The task is reconstruction.

Start with how you acquired the property

The first question is not always, “What did I pay?”

It is:

How did I acquire this asset?

Different acquisition methods can invoke different basis rules.

Purchased property generally begins with cost under IRC §1012.

Inherited property can fall under the inherited-basis rules of IRC §1014.

Gifted property can fall under IRC §1015.

Employer stock, reorganizations, and other substituted-basis transactions can require their own analysis.

That is why “cost basis” is useful shorthand for ordinary purchased securities but should not be treated as a universal description of every investment's basis.

Once the correct initial basis is established, the next task is determining whether later events changed it.

Initial basis is not always adjusted basis

An asset can begin with one basis and be sold years later with another.

Relevant basis-changing events can include:

• stock splits;
• nondividend distributions;
• mergers and reorganizations;
• spin-offs;
• wash-sale adjustments; and
• other adjustments required under IRC §1016.

This is why an original purchase confirmation may be necessary but insufficient.

The tax calculation generally needs the adjusted basis at disposition, not merely the amount originally paid.

Example: reconstructing adjusted basis

Suppose Form 1099-B reports $20,000 of proceeds but shows no basis.

Historical records establish that the investment was originally purchased for $12,000.

The taxpayer also establishes that $2,000 of nondividend distributions reduced basis over the holding period.

Before considering any other adjustments:

Original basis: $12,000
Basis reduction: $2,000
Adjusted basis: $10,000

The blank broker field did not turn the basis into zero. The historical facts produced a $10,000 adjusted basis.

At that point, this page's job is largely finished.

How that basis is reconciled with broker reporting on the return belongs in Form 1099-B and Form 8949.

What evidence can establish basis?

Basis reconstruction is often an evidence problem.

Useful records can include:

• historical purchase confirmations;
• monthly and year-end brokerage statements;
• transfer statements;
• dividend reinvestment histories;
• issuer records;
• Forms 8937 for organizational actions;
• merger and spin-off tax notices;
• probate and estate records;
• Schedule A of Form 8971 where applicable;
• gift records;
• employer equity records;
• prior tax returns and Forms 8949; and
• wash-sale workpapers.

No single document necessarily resolves the entire history.

For example, an issuer's Form 8937 can be highly useful in understanding a basis-changing corporate action without proving every basis event that occurred before or after that action.

Reconstructing basis does not let you choose a tax lot after the sale

This is another important boundary.

Suppose you determine that you bought identical shares on several dates at several prices.

Reconstructing the basis of each lot tells you what those lots are.

It does not automatically tell you which lot was sold.

And it does not create a retroactive right to choose the most favorable lot when preparing the return.

That is a separate legal question governed by the lot-identification rules.

See Specific Identification vs. FIFO.

The sequence should remain:

Missing or Wrong Cost Basis: What was the correct adjusted basis of each position?
→ Specific Identification vs. FIFO: Which position was actually sold?
→ Form 1099-B and Form 8949: How is that result reconciled with broker reporting?

The distinctions are deliberate.

What if the broker's number is actually wrong?

First establish the substantive answer.

Do not begin by asking what number would produce the preferred tax result. Begin with the acquisition history and basis adjustments.

Once the correct adjusted basis has been established, compare it with the broker-reported information.

If the two differ, the tax return may need to reconcile the difference. The exact reporting mechanics belong in Form 1099-B and Form 8949, not here.

Keep the acquisition date too

A basis reconstruction should not stop at the dollar amount.

The acquisition date can affect:

• short-term versus long-term character;
• identification among multiple lots; and
• holding-period tacking in transactions where special rules apply.

A reconstructed position should therefore have both a supportable adjusted basis and a supportable acquisition/holding-period history.

When reconstruction becomes difficult

The legal rules can be clearer than the facts.

A security owned for decades may have moved through multiple brokers, dividend reinvestment plans, splits, mergers, spin-offs, and distributions. An inherited or gifted asset may arrive without the records needed to establish its history.

In those cases, it is important to distinguish between:

a basis established by the available records;
a defensible reconstruction from reliable evidence; and
an unsupported estimate.

Those are not the same thing.