Forty three subfertile male patients exhibiting a wide range of sperm counts received Tamoxifen (Nolvadex, I.C.I.) 20 mg daily for six months continuously; nine pregnancies resulted, a rate of 21 per cent. Although a significant improvement in sperm output occurred in three patients with initial sperm counts of less than 20 x 10(6)/ml the response was most marked in those with an initial count of less than 5 x 10(6)/ml and four pregnancies occurred in this latter group. Patients whose initial count was greater than 20 million per ml showed no significant response and four exhibited a steady fall in sperm output. No correlation was found between the response to treatment and basal serum levels of FSH, LH, testosterone and prolactin. Four markedly oligozoospermic patients with a significantly elevated FSH, normally associated with severe tubular dysfunction, showed a pronounced increase in sperm count. Seven patients (16 per cent) experienced minor side effects.