The main objectives of this study were to identify the determinants of condom use among HIV-positive men having anal sex with HIV-negative men or men of unknown HIV status and moderators of the intention-behaviour relationship. A cohort of 237 sexually active HIV-positive men having sex with men was followed over a period of six months. The cognitive variables measured were guided by an extended version of the theory of planned behaviour. Results indicated that past behaviour (odds ratio [OR] = 9.75; 95% confidence interval [CI]: 4.48-21.26), intention (OR = 3.13; 95% CI: 1.25-7.81), self-efficacy (OR = 3.62; 95% CI: 1.40-9.37) and use of sex drugs (OR = 0.16; 95% CI: 0.06-0.45) contributed to the prediction of 100% condom use. Self-efficacy also interacted with intention as a significant moderator of the intention-behaviour relationship (OR = 20.96; 95% CI: 2.90-151.51). Interventions promoting condom use should increase self-efficacy to use condoms and target users of sex drugs.