The clinicopathologic features of neoplasms arising in gastrointestinal endometriosis have not been well characterized. In this series, we report 17 cases of gastrointestinal endometriosis complicated by neoplasms (14 cases) or precancerous changes (three cases). Four patients, one of whom also had hypermenorrhea, presented with chronic abdominal pain and five had obstructive symptoms; one of these also had rectal bleeding. One patient presented with an acute abdomen and fecal peritonitis, one had vaginal bleeding, and one had a progressive change in bowel habits. Nine patients had a long history of endometriosis, 11 patients had had hysterectomies, and eight of these had also received unopposed estrogen therapy. The lesions involved the rectum (6), sigmoid (6), colon, unspecified (2), and small intestine (3), and comprised 8 endometrioid adenocarcinomas (EA), 4 mullerian adenosarcomas (MAS), 1 endometrioid stromal sarcoma (ESS), 1 endometrioid adenofibroma of borderline malignancy (EBA) with carcinoma in situ, 2 atypical hyperplasias (AH), and one endometrioid adenocarcinoma in situ (ACIS). The tumors ranged in size from 2 to 15 cm and all involved the serosa and muscularis propria. Two tumors extended into the mucosa, with mucosal ulceration in one. Follow-up was available in 11 cases. One patient with EA was dead of disease at 1 year, one had two recurrences at 1 and 2 years, and three were alive with no evidence of disease (ANED) at 9 months to 13 years (mean, 68 mos). The patient with the EBA was ANED at 3 months. Two patients with MAS were ANED at 2 and 3 years. The patient with ESS had a recurrence at 3 years and was ANED 6 years after her original diagnosis. One woman with AH was ANED at 60 months and the patient with ACIS was ANED at 16 months. One of the carcinomas was originally misdiagnosed as a primary intestinal adenocarcinoma. The pathologist should be aware of the possibility of a tumor of genital tract type when evaluating intestinal neoplasms in females, particularly if they have a history of endometriosis and have received unopposed estrogen therapy.