Objectives: A high incidence of HIV continues among men who have sex with men (MSM) in industrialised nations and research indicates many MSM do not disclose their HIV status to sex partners. Themes as to why MSM attending sexually transmitted infection (STI) clinics in Los Angeles and Seattle do and do not disclose their HIV status are identified. Methods: 55 HIV positive MSM (24 in Seattle, 31 in Los Angeles) reporting recent STI or unprotected anal intercourse with a serostatus negative or unknown partner from STI clinics underwent in-depth interviews about their disclosure practices that were tape recorded, transcribed verbatim, coded, and content analysed. Results: HIV disclosure themes fell into a continuum from unlikely to likely. Themes for "unlikely to disclose" were HIV is "nobody's business," being in denial, having a low viral load, fear of rejection, "it's just sex," using drugs, and sex in public places. Themes for "possible disclosure" were type of sex practised and partners asking/disclosing first. Themes for "likely to disclose" were feelings for partner, feeling responsible for partner's health, and fearing arrest. Many reported non-verbal disclosure methods. Some thought partners should ask for HIV status; many assumed if not asked then their partner must be positive. Conclusions: HIV positive MSM's decision to disclose their HIV status to sex partners is complex, and is influenced by a sense of responsibility to partners, acceptance of being HIV positive, the perceived transmission risk, and the context and meaning of sex. Efforts to promote disclosure will need to address these complex issues.