Objective: To assess predictors of relapse into unprotected anogenital intercourse with casual partners among homosexual men in Amsterdam, The Netherlands. Design: A prospective study ongoing since October 1984. Methods: Self-reported data on sexual behaviour were obtained at 12 semi-annual intervals. Participants who reported unprotected anogenital intercourse with casual partners at wave 12 but not at wave 11 were considered to have relapsed into unsafe sex (n = 47). Men who did not report unprotected anogenital intercourse at wave 11 or at wave 12 were considered to have maintained behaviour change (n = 197). Results: Relapse into unsafe sex with casual partners occurred more frequently among men with a less positive attitude towards condom use, lower personal efficacy with respect to using condoms with casual partners, a weaker intention to avoid anogenital intercourse with casual partners and those who were not involved in a primary relationship. Intention to avoid anogenital intercourse with casual partners was related to an individual's attitude, normative beliefs and personal efficacy with respect to avoiding anogenital intercourse. Conclusions: In order to improve motivation it is important to support homosexual men in their personal decision-making process, which may be more resolute when the outcome is in accordance with individual preferences. Therefore, health education should be community-based and sensitive to individual needs.