We studied 32 heroin and methadone addicts, divided into 4 groups according to the type of drugs used: 5 heroin-dependent, 10 taking methadone plus heroin more or less constantly, 10 taking methadone plus heroin occasionally, and 7 taking methadone only. 93% of the heroin addicts and 65% of those taking methadone had abnormal semen. The most frequent abnormality was in motility (78%). Teratozoospermia was the second most frequent (28%) and oligozoospermia the third (16%). The 7 patients taking methadone only had neither teratozoospermia nor oligozoospermia. This semen pathology is probably secondary to decreased testosterone production, with relative hypofunction of the seminal tracts and the accessory glands.