Five patients were treated with gonadotropin and topical testosterone for micropenis associated with hypothalamic hypogonadotropic hypogonadism. All patients received 1,000 units of gonadotropin weekly for 3 weeks, with a 6-week interval followed by 10% topical testosterone cream twice daily for 3 weeks. Serum testosterone levels were measured and remained equivalent for both modes of therapy. Average penile growth response with gonadotropin was 14.3% increase in length and 5.0% increase of girth. Topical testosterone produced an average increase of 60% in penile length and 52.9% in girth. The greatest growth response occurred in prepubertal male subjects with a minimal response in postpubertal male subjects. This study suggests that 10% topical testosterone cream twice daily will produce effective penile growth. The response appears to be greater in younger children, which is consistent with previously published studies of age-related 5 reductase activity.