Purpose: Researchers are increasingly using national population surveys (NPS) to understand the health of gay and bisexual men (GBM). However, valid inference from these surveys depends on accurate identification of GBM. Methods: We asked 8126 GBM in an anonymous, online, community-recruited survey about their willingness to reveal their sexual orientation in NPS. Results: Overall, 30% indicated that they would not reveal their sexual orientation; however, there were differences in frequencies according to sexual orientation, relationship status, age, HIV status, living environment, education, income, and ethnicity. Conclusion: NPS might not be fully representative of GBM due to misclassification errors stemming from unwillingness to disclose.