Thirty cases of ovarian endometrioid tumor of low malignant potential (ETLMP) were studied and compared with 32 cases of well-differentiated endometrioid adenocarcinoma. ETLMP was distinguished from well-differentiated endometrioid adenocarcinoma by the absence of destructive stromal invasion, glandular confluence, or stromal disappearance. Intraepithelial carcinoma in a low malignant potential tumor was defined as areas showing grade 3 nuclei, sometimes associated with an intracystic villoglandular or cribriform pattern. Microinvasion in an ETLMP was defined as one or more areas of invasion with an area of < or =10 mm2. Because a cribriform pattern may be seen in purely intraglandular proliferations, the latter was not taken as evidence of invasion. The patients with ETLMP ranged from 28 to 86 years of age (mean 54.9 years), and only one patient (3%) had other than stage I disease at presentation. The patients with well-differentiated endometrioid carcinoma ranged from 26 to 87 years of age (mean 51.1 years), and three patients (9%) had stage II disease at presentation. An adenofibromatous pattern was present in 47% of cases of ETLMP and squamous differentiation in 47%; intraepithelial carcinoma occurred in 7% of cases and stromal microinvasion in 7%. None of these findings appeared to influence the prognosis because all patients with ETLMP were free of recurrent disease or metastasis on follow-up, whereas 20% of patients with well-differentiated endometrioid adenocarcinoma followed for >6 months developed recurrent disease. Thus, the prognosis of ETLMP, when defined by the above criteria, is favorable and is superior to that of well-differentiated endometrioid adenocarcinoma.