Thirty-three subfertile men with idiopathic normogonadotropic oligozoospermia were treated with daily 2 x 10 mg tamoxifen, a nonsteroidal antiestrogen, for a period of five months. The statistical evaluation of the semen parameters yielded a significant increase of the sperm count and the total sperm output after three months of treatment with a further increase after five months. The percentage of motile spermatozoa increased already after one month and remained significantly increased during the whole treatment period. Progressive motility, sperm morphology, ejaculate volume and seminal plasma fructose were not affected. Side effects were not observed. The conception rate was 38 per cent within a period of one year.