OBJECTIVE To test the hypothesis that spontaneous endometriosis is a progressive disease in baboons. METHODS Prospective cohort study, Institute of Primate Research, Nairobi, Kenya. METHODS Twelve baboons with histologically proven spontaneous endometriosis. METHODS Fifty-four serial laparoscopies (4.2 +/- 1.9 per animal, range 1 to 7) were performed after 1 to 3 months (n = 2), 4 to 6 months (n = 1), 7 to 9 months (n = 6), 10 to 12 months (n = 17), 13 to 15 months (n = 7), 16 to 18 months (n = 1), 19 to 21 months (n = 8), 22 to 24 months (n = 8), 25 to 27 months (n = 2), and 30 to 32 months (n = 2). METHODS During each laparoscopy the pelvis was examined for the presence of endometriosis. The number, size, and type of endometriotic implants were noted on a pelvic map and both endometriosis score and stage were calculated. Remodeling was defined as a change in laparoscopic appearance (typical, subtle, suspicious) of an individual endometriotic lesion. Data were analyzed by a signed rank test. RESULTS Endometriosis was progressive as shown by the significant increase in the number and surface of lesions and in endometriosis scores after 24 months. The total number of endometriotic lesions after 24 months consisted of 69% new (and mostly subtle) implants, 10% remodeled lesions, and 21% unchanged implants. CONCLUSIONS This study demonstrated that endometriosis in captive baboons undergoing repeated laparoscopies is a dynamic and moderately progressive disease with periods of development and regression and active remodeling between different types of lesions.