Knowledge of the determinants of lapses into unsafe sex are important parts of HIV prevention interventions. The present study examines the determinants of lapses into unsafe sex, and the role of attribution in predicting lapses into unsafe sex within an HIV prevention treatment program for homosexual men based on the relapse prevention (RP) model (Marlatt & Gordon, 1985). Self-reported negative emotional states and general urges to engage in sex were perceived to precede violations of safer sex goals. Stable and global attributions for a goal violation related to the probability of a second concurrently assessed violation. Prospective analyses indicated that future unprotected oral and anal sex was predicted by current unprotected oral and anal sex. In addition, more stable and external attributions for previous goal violations added to the prediction of future unprotected anal sex. Results are discussed in relation to the RP model, the role of attributions in safer sex goal violations, and treatments to reduce unsafe sexual behavior.