Leiomyomatosis peritonealis disseminata is a pseudomalignant condition which mimics disseminated abdominal carcinoma. A new case is reported and compared to the 11 well-documented cases in the literature. All reported patients have experienced a benign clinical course. The peritoneal nodules are composed of histologically benign-appearing smooth-muscle cells, substantiated by electron-microscopic study. The histogenesis of this entity is discussed in the light of our ultrastructural findings and the observation of concurrent endometriosis. The theory of metaplasia of the subcoelomic mesenchyme in the pathogenesis of the leiomyomatous lesions is favored. The serum estrogen levels determined in our patient did not reveal any abnormal change. The inducing factor(s) remain to be identified. Also, an unusual sensitivity of the coelomic tissues in these patients, in response to metaplastic stimuli, is postulated as a possible contributing factor.