This study was performed to test the hypothesis that endometriosis undergoes regression during pregnancy.This study was performed on 11 baboons with histologically proven endometriosis, housed at the Institute of Primate Research, Nairobi, Kenya. In each individual baboon paired laparoscopies were performed prior to and during pregnancy (6 during first and 5 during second trimester of gestation) with an interval of 5 +/- 3 months. During each laparoscopy the number, size and type of endometriosis implants were noted in detail on a pelvic map; the endometriosis score and stage were calculated according to the revised American Fertility Society (AFS) classification. In each baboon the observations prior to and during pregnancy were compared and analysed by Wilcoxon signed rank test (two-tailed).No significant change was observed in the AFS score or stage of endometriosis, or in the number, size and type of endometriotic lesions in baboons during gestation when compared to the nonpregnant state.In baboons pregnancy had no significant effect on endometriosis during the first or second trimester of gestation.