Some studies have suggested that growth hormone (GH) may enhance folliculogenesis in women, and similarly may enhance spermatogenesis in men with hypogonadotrophic hypogonadism. In this prospective open-controlled pilot study, we investigated the effect of daily subcutaneous GH for 5 months in 12 endocrinologically normal men with severe idiopathic oligozoospermia (< 10 million/ml). All the men had normal karyotype and endocrine tests, including a GH response of > 20,000 mU/l to insulin hypoglycaemia. Nine men with similar sperm counts acted as controls. During treatment, each patient was examined monthly, asked for side effects and had glycosylated haemoglobin, glucose and blood counts monitored. Five semen samples were obtained in the 4 months before treatment, two samples per month during treatment and three samples after stopping treatment. The mean insulin-like growth factor I (IGF-I) was normal before treatment and 1 month after ending treatment, at 206 and 182 micrograms/l, respectively, but increased significantly during treatment to 444 micrograms/l (p < 0.0001, ANOVA). The mean (SD) sperm counts were 2.6 (2.5), 2.5 (3.7) and 2.3 (2.1) million/ml before, during and after GH treatment, respectively, and did not show any statistically significant differences (ANOVA). We conclude that GH does not increase or decrease sperm counts in men with severe idiopathic oligozoospermia.