Multilocular cysts of the peritoneum diagnosed at the Mayo Clinic from 1907 through 1981 were examined and divided, on the basis of their light microscopic features, into 2 groups-cystic mesotheliomas and cystic lymphangiomas. There are apparent clinical differences between the 2 groups. Cystic mesotheliomas tend to occur in adult females and often recur. Cystic lymphangiomas occur most often in males, often develop in children, and rarely recur, if at all. The distinguishing histologic features are the cytologic characteristics of the cells lining the cysts and the presence of absence of smooth muscle in the wall of the cysts.