We used the aromatase inhibitor testolactone (40 mg/kg.day) to treat 12 girls with precocious puberty due to the McCune-Albright syndrome for periods of 0.5-5 yr. In the 7 girls who received testolactone for at least 3 yr, the mean +/- SD serum estradiol level was 618 +/- 268 pmol/L at the start of therapy and fell to 156 +/- 84 pmol/L at 1 yr, 116 +/- 48 pmol/L at 2 yr, and 241 +/- 260 pmol/L at 3 yr (P < 0.05 compared to the start of therapy), with recurrent ovarian cysts at 3 yr in 2 patients. These 7 girls averaged 8 menses/yr before therapy. The average frequency of menses decreased to 2 episodes/yr during the first year of treatment, 3/yr during the second year, and 4/yr during the third year. The mean +/- SD testosterone levels were slightly above the normal prepubertal range (0.51 +/- 0.2 nmol/L) before treatment and did not change significantly during treatment. The mean +/- SD androstenedione levels rose from 1.1 +/- 0.6 nmol/L before treatment to 2.1 +/- 0.1 nmol/L at 2 yr and 2.8 +/- 0.1 nmol/L after 3 yr of treatment (P < 0.05 compared to before treatment) and were consistent with normal adrenarche. The mean predicted adult stature was 143.0 +/- 7.8 cm before treatment and 147.3 +/- 11.5 cm at 3 yr (P = NS). In 3 of 12 girls, all with bone age greater than 12 yr, the gonadotropin responses to LHRH indicated early central precocious puberty after 1-4 yr of treatment. The adverse effects of testolactone were transient abdominal pain, headache, and diarrhea in 3 girls and elevated hepatic enzymes in 1 girl who had abnormal liver function before treatment. Six families acknowledged difficulty in adhering to the daily dosing schedule. We conclude that testolactone can be effective in the treatment of LHRH-independent precocious puberty in girls with McCune-Albright syndrome, but that some patients exhibit an escape from the effects of treatment after 1-3 yr.