Leiomyomatosis peritonealis disseminata (LPD) is a rare condition that primarily affects women of reproductive age. Immunohistochemical studies were performed in four cases: LPD from a premenopausal woman on oral contraceptives (one case); LPD associated with postpartum massive ectopic decidual reaction (one case); and LPD from a perimenopausal and a postmenopausal woman. Progesterone receptor activity was present in nine of nine cases, and eight of eight cases were strongly positive for vimentin; reactivity for cytokeratin was uniformly negative. Most cases had a pattern of staining typical of smooth muscle tumors with expression of desmin, smooth muscle actin, and muscle-specific actin. Although estrogen receptor was detected in most cases, reactivity was notably absent (one case) or weak (one case) in nodules with a prominent decidual reaction. Expression of CD 34, a marker for which LPD staining characteristics have not been previously reported, varied from absent to weak. Peritoneal nodules from the postmenopausal woman lacked staining for both estrogen receptor and desmin, smooth muscle actin and muscle-specific actin were only focally expressed, whereas staining for CD 34 was focally intense. Uterine myometrium and leiomyomata were positive for progesterone and estrogen receptor, vimentin, desmin, smooth muscle actin, and muscle-specific actin. Cytokeratin expression was absent. CD 34 exhibited weak staining in leiomyomata, but was absent from myometrium. Progesterone receptor appears to be uniformly expressed in LPD nodules from premenopausal and postmenopausal women, a finding supporting the contention that hormones influence the development of LPD in all cases, regardless of menopausal status.