The present study details gross and histologic findings of 79 previously ligated fallopian tubes from 3 groups of patients. Of 20 oviducts removed after documented sterilization failure (group I), 6 revealed a process compatible with endometriosis. Four of nine previously ligated fallopian tubes removed at the Johns Hopkins Hospital (group II) were successfully injected with India ink. In two patients, histologic examination demonstrated the India ink in epithelium-lined spaces that lay beyond the muscle of the tubal wall extending from the tubal lumen to the serosal surface. 50 oviducts were studied in 25 patients requesting reversal of their sterilizations (group III). A higher percentage of fistulas was demonstrated in patients with less than 4 cm of remaining proximal tubal segment. Furthermore, most of these fistulas were demonstrated in patients for whom 3 years had elapsed since the original sterilization procedure. Patients sterilized by laparoscopic cautery methods were observed to have a higher percentage of fistula formation and histologic documentation of endometriosis at the sterilization site as compared with patients sterilized by other methods. Our observations suggest that ligation of the oviduct within 4 cm of the uterine cornu may predispose to the development of endometriosis and subsequent fistula formation in the tip of the ligated oviduct.