ObjectiveTo assess predictors of relapse into unprotected anogenital intercourse with casual partners among homosexual men in Amsterdam, The Netherlands.DesignA prospective study ongoing since October 1984.MethodsSelf-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).ResultsRelapse 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.ConclusionsIn 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.