22 Boys with pubertal gynaecomastia (age 15.9 +/- 1.9 years) were treated with testolactone (450 mg daily by mouth) for 2 to 6 months without side-effects. The mean breast gland diameter regressed from 4.4 to 3.3, 3.2 cm, and 1.7 cm at 2, 4, and 6 months, while pubic hair and testicular volume progressed normally. Plasma androstenedione increased from 5.4 to 73.1 nmol/l. Testosterone, DHEA, and oestrone increased less, and oestradiol remained unchanged. Androgen/oestrogen ratios increased (most marked change: androstenedione/oestrone from 15 to 140). LH (basal and maximum after LHRH) did not change, but FSH increased somewhat (basal 133 to 173, maximum 225 to 269 micrograms/l). Prolactin remained unchanged. It is concluded that testolactone, an inhibitor of steroid aromatization, is an effective and safe medical treatment for pubertal gynaecomastia.