The clinical and histological efficacy of gestrinone in the treatment of endometriosis was evaluated. A prospective study was performed with longitudinal follow-up of 4 years duration (minimum) for each patient. Twenty-five cases of endometriosis were treated with gestrinone (as a drug of choice or associated with other local surgical treatment) during 6 months. The total or partial response to pain was induced with treatment in 19 of the 20 cases (95%), and posttreatment pregnancies were induced in 3 of the 9 cases of infertility (33.3%). The global clinical efficacy was as follows: none (n = 5, 20%), poor (n = 2, 8%), moderate (n = 2, 8%), good (n = 3, 12%) and excellent (n = 13, 52%). The pretreatment American Fertility Society score was 23.1 +/- 3.8 points (2-58) and that after treatment (re-examination) 7.8 +/- 3.7 points (0-52), p < 0.01. Global histological efficacy (n = 17) results were as follows: none (23.6%), poor (0%), moderate (5.8%), good (35.3%) and excellent (35.3%). Gestrinone is presented as a drug with an efficacy comparable to other pharmacological agents currently available in the medical treatment of endometriosis. Adequate posology and acceptable effectiveness were also observed.