When may an expert disclose the underlying facts or data supporting their opinion?

• A. Only after cross-examination about the opinion.
• B. After disclosure of the underlying facts or data.
• C. Never; only the opinion is admissible.
• D. Only if requested by the opposing party in writing.

Answer: (B)

The main idea is that an expert’s opinion rests on facts or data they rely on, and those underlying materials should be available to the other side so the basis for the opinion can be tested. The correct choice reflects that disclosure happens after the underlying facts or data have been disclosed to the opposing party. In practice, you typically reveal or produce the data upon request or during cross-examination, once the opposing side has access to them, so they can evaluate whether the opinion rests on a solid foundation. This ensures fairness and allows scrutiny of the opinion’s basis. The other options don’t fit because disclosure isn’t limited only to cross-examination, nor is it restricted to a written request, and it isn’t true that the underlying data can never be disclosed.