224 infertile men were treated with mesterolone (M.;30, or 60 mg/die) for a minimum of three months. When comparing semen analyses done before and after treatment with M., a small but distinct reduction of hyperspermia as well as an augmentation of hypospermia were evident, an increase of sperm density in oligozoospermia grade I (20-40 kx 10(6) sperm/ml) was revealed, and an improvement of sperm motility in asthenozoospermia was seen. These was no change in sperm morphology. According to our data, careful selection of patients for treatment with M. Based on aetio-pathogenic considerations appears to be the "salient point" in the administration M. As a general rule, treatment with M. should be confined to patients with mild idiopathic oligo (astheno) zoospermia where no specific therapy - such as, e.g., high ligation for varicocele - is available.