Erectile dysfunction (ED) is common among people in treatment for heroin addiction. The purpose of the study was to examine the frequency of ED among methadone and buprenorphine maintenance therapy patients, and to identify factors associated with ED. Patients - recruited from 7 centres in Italy - underwent: (i) a structured interview on socio-demographic characteristics, drug use and sexual behaviour; (ii) IIEF-15 test, a test of sexual function; (iii) Zung test for depression. The study included 201 males: 42% were on methadone maintenance, 58% were on buprenorphine. Overall, 58% reported no ED, 24% reported mild to moderate ED, and 18% severe ED. In univariate analysis buprenorphine patients had less ED than methadone patients (p=0.0135). Subjects living with a partner had less ED than others (p=0.0018). More depressed subjects had more ED (p<0.001). Heterosexual patients reported less ED than homo/bisexual patients (p=0.0427), and partner's use of heroin was associated with more ED (p=0.0078). The significant univariate predictors were entered into a cumulative logit model. Living with a sexual partner was associated with a lower likelihood of ED, while depression, having a sexual partner with a history of drug use and not having a steady partner were associated with a greater likelihood of ED. The significant association between treatment and ED which appeared in univariate analysis (with buprenorphine patients reporting less ED than methadone patients) was not confirmed by the multivariate analysis. Both psychological and social factors were associated with ED which is an important problem for many males in methadone and buprenorphine treatment.