TO AVOID social isolation, discrimination, or verbal or physical abuse, many men who have sex with men (MSM), especially young and minority MSM, do not disclose their sexual orientation.^1-3 Young MSM who do not disclose their sexual orientation (nondisclosers) are thought to be at particularly high risk for human immunodeficiency virus (HIV) infection because of low self-esteem, depression, or lack of peer support and prevention services that are available to MSM who are more open about their sexuality (disclosers).^1-3 However, the risks for HIV infection and other sexually transmitted diseases (STDs) are unknown for nondisclosers. To better understand the prevention needs of young MSM, CDC analyzed data from the Young Men's Survey (YMS) to compare HIV/STD risk differences between nondisclosers and disclosers. This report summarizes the results of that analysis, which indicate that 8% of 637 nondisclosers were infected with HIV compared with 11% of 4,952 disclosers. Among blacks, the prevalence of HIV infection was 14% among 199 nondisclosers compared with 24% among 910 disclosers. Compared with disclosers, nondisclosers had similar high risks for other STDs, reported less sexual behavior with men and more sexual behavior with women, reported less use of HIV testing services, and, among those who were HIV infected, were less likely to be aware of their infection. To reduce HIV/STD transmission among young MSM and their female sex partners, comprehensive HIV/STD testing and prevention programs for young nondisclosers, especially for those who are black, should be developed or expanded.