The clinical and pathological features of four unusual cases of pelvic endometriosis are described. The patients, who were between the ages of 40 and 72 years, underwent operations for uterine leiomyomas in two cases, endometrial adenocarcinoma in a third case, and an enlarging abdominal mass in a fourth case. In three patients, two of whom had enlargement of one or both ovaries, multiple nodules were either attached to the peritoneum (two cases) or lay free in the peritoneal cavity (one case). In one of these cases, the intraoperative findings mimicked metastatic carcinoma. Histological examination of the peritoneal lesions, as well as the ovaries in all the cases, revealed granulomatous nodules composed of a central focus of necrotic tissue surrounded by histiocytes (pseudoxanthoma cells), hyalinized connective tissue, or both. Endometriotic glands and stroma were typically absent in the nodules and their immediate vicinity. Foci of recognizable endometriosis, however, were found in the ovaries in all the cases. In one of the ovaries, several necrotic nodules lined an endometriotic cyst; in another, a focus had an appearance transitional between endometriosis and a necrotic nodule. These necrotic pseudoxanthomatous nodules are a previously undescribed manifestation of endometriosis; they occur characteristically in older women. On histologic examination, they should be distinguished from other necrotic peritoneal and ovarian granulomas, as well as necrotic tumor.