Twenty-five cases of multilocular peritoneal inclusion cysts (MPIC) were investigated. All but four cases were associated with a history of a previous abdominal or pelvic operation, or evidence of endometriosis or pelvic inflammatory disease, or combinations of these findings. All of the lesions were attached to pelvic organs, 44% also occupied the upper abdominal cavity, and 16% involved the retroperitoneum. In three cases free-floating cysts were present as well. The median diameter of the lesions was 13 cm. The cyst locules were lined by one to several layers of flat to cuboidal mesothelial cells that occasionally formed papillae, had a hobnail shape, or had undergone squamous metaplasia. The stroma was characterized by chronic inflammation and often acute inflammation. In most of the cases there was mural proliferation of the mesothelial cells, occasionally simulating a malignant mesothelioma. Twelve lesions were complicated by postoperative local recurrence; in four of these cases the recurrences were multiple; neither the size of the lesion nor the presence of mural mesothelial proliferation influenced the outcome. The clinical and pathologic data in this series suggest that the MPIC is a nonneoplastic reactive mesothelial proliferation.