The purpose of this study was to evaluate efficacy and safety parameters in women with leiomyomata uteri treated with the GnRH agonist leuprolide acetate depot, 3.75 mg intramuscularly every 4 weeks for 24 weeks. One hundred twenty-eight patients were enrolled in a randomized, double-blind, placebo-controlled multicenter study involving 13 investigative centers. Mean uterine volume decreased by 36% at 12 weeks and 45% at 24 weeks of leuprolide therapy. Patients treated with placebo had increases in mean uterine volume of 16% at 12 weeks and 5% at 24 weeks. Seventyseven percent of leuprolide-treated patients had a more than 25% reduction in uterine volume, compared with 9% of placebo-treated controls. Mean uterine volume returned to pre-treatment size 24 weeks after cessation of leuprolide treatment. The majority of patients had resolution or improvement of their fibroid-related symptoms after 24 weeks of leuprolide treatment. Of 38 leuprolide-treated patients presenting with menorrhagia, 37 (97%) had resolution of this symptom at the time of the final visit. Although 95% of women treated with leuprolide acetate experienced some side effects related to hypoestrogenism, only five patients (8%) terminated treatment prematurely. We conclude that leuprolide acetate depot treatment of leiomyomata uteri is safe and causes significant but temporary reductions in uterine size and fibroid-related symptoms.