Extrauterine formation of decidua of stromal cells has been well described, particularly in the cervix and ovary. Sporadic reports have documented decidua formation of peritoneal surfaces and lymph nodes as well as ovarian ectopic decidua in pregnant patients. The apparent hormonal mechanism of this phenomenon suggests a relationship to endometriosis. Between 1983 and 1986, tissue from 10 pregnant patients (acquired from nine at cesarean section and from one at appendectomy) demonstrated submesothelial decidua formation in the form of microscopic nodules and diffuse cell arrangements in an edematous stroma. Some cases showed mild chronic inflammation. An intracytoplasmic lipofusin-type pigment was observed in one case. Four cases had diffuse omental involvement: two with small amounts of free peritoneal blood and two with peritoneal adhesions. Three cases were associated with paratubal cysts: two of these occurred in the uterine and appendiceal serosa, respectively, and one case showed involvement of a retroperitoneal lymph node associated with a pheochromocytoma. No specific gross observations were noted at surgery. There was no prior or subsequent evidence of endometriosis. A review of 958 elective tubal ligations performed between 1983 and 1985 demonstrated 52 examples (5.5%) of serosal decidua formation. Ectopic stromal decidua formation in pregnancy is: (a) a physiologic phenomenon related to the possible specialized sensitivity of the superficial coelomic stroma to progesterone; and (b) diffusely distributed in the peritoneum although it is a clinicopathologic process distinct from endometriosis.