There is no conclusive evidence of the usefulness of tamoxifen in the treatment of idiopathic oligozoospermia (OAT-syndrome), as it has been used mostly in uncontrolled studies. We herein report on the controlled treatment of OAT-syndrome with tamoxifen versus placebo following a randomized design. Seventy-six men with sperm counts of 2-20 x 10(6) ml-1, sperm motility of 20-50%, and sperm morphology (abnormal cells) between 50 and 80% were involved in the study. Patients with varicocele, a history of testicular maldescent or genital inflammation were excluded. Thirty-nine patients received tamoxifen (30 mg daily), 37 patients placebo. There was a statistically significant increase in the mean serum testosterone level after treatment in the tamoxifen-treated group (from 4.9 +/- 1.9 to 7.9 +/- 3.6 ng ml-1) in comparison to the placebo group (5.3 +/- 2.0 and 5.6 +/- 2.0 ng ml-1). Serum FSH levels increased slightly in the tamoxifen group (from 6.8 +/- 4.1 to 7.3 +/- 4.8 mU ml-1), but this was not statistically significant in comparison to the placebo group (from 5.9 +/- 3.9 to 5.2 +/- 3.5 mU ml-1). Serum levels of LH did not show any differences between groups. The sperm count increased during treatment from 9.3 +/- 11.7 to 11.4 +/- 13.7 x 10(6) ml-1 in the tamoxifen group and from 9.1 +/- 7.1 to 9.3 +/- 8.8 x 10(6) ml-1 in the placebo group; this difference did not reach statistical significance. The percentage of motile and abnormal sperm was not different between the two treatment groups.(ABSTRACT TRUNCATED AT 250 WORDS)