Epidermal growth factor (EGF) content in seminal plasma of 17 patients with androgen deficiency (12 Klinefelter's syndrome and 5 hypogonadotropic hypogonadism) was determined before and after hormone replacement. No significant increase of plasma testosterone level was demonstrated after testosterone administration in Klinefelter's syndrome, although the level in 4 of 5 patients with hypogonadotropic hypogonadism reached the normal range. In 16 patients, except 1 with hypogonadotropic hypogonadism with no increase of blood testosterone level, prostate volume increased after hormone therapy, and volumes after treatment were significantly larger than those before treatment. On the other hand, EGF concentration in seminal plasma remained at the pretreatment level in most patients. It is therefore concluded that although the prostate volume increases to almost normal after treatment, prostate function does not improve in most patients, indicating that in patients with hypogonadotropic hypogonadism fertility potential may remain low, even if they obtain normal seminal parameters after treatment.