To compare the potency, side effects, and duration of action of triptorelin and leuprorelin acetate after i.m. injections.Prospective, double-blind crossover clinical study.A teaching hospital.Fifty-four patients with pelvic endometriosis.Twenty-seven patients had three doses of i.m. triptorelin (3.75 mg) followed by three doses of i.m. leuprorelin acetate at 4-week intervals. Twenty-one patients had three doses of i.m. leuprorelin acetate (3.75 mg) followed by three doses of i.m. triptorelin, also at 4-week intervals.Menopausal symptoms, time taken for menstruation to return, serum E(2), FSH, LH levels, lipid profiles, and liver function tests.The potencies of triptorelin and leuprorelin acetate in lowering the serum E(2), FSH, and LH levels were comparable. The severity of menopausal symptoms, changes in the lipid profile and liver function parameters were similar after triptorelin and leuprorelin acetate. The resurgence of ovarian activities and the spontaneous return of menstruation occurred significantly earlier after leuprorelin acetate than triptorelin.Both drugs are equally potent in down-regulating the pituitary-ovarian function, and their side effects are similar. Triptorelin has a longer duration of drug action and can be administered over a longer interval period.