We present the results of a clinicopathologic study of 20 patients with primary extrauterine endometrial stromal sarcoma (ESS). The sites of the primary neoplasm and the number of patients with sufficient follow-up for survival analysis are as follows: ovary (three of four), fallopian tube (one of one), pelvic cavity (six of eight), abdominal cavity (five of six), and retroperitoneum (one of one). Evaluation of all patients included the mitotic index (MI) and cytologic atypia. Thirteen of the sixteen patients eligible for survival analysis had tumors with an MI < 10 and would be classified as low-grade stromal sarcomas in the Morris and Taylor scheme. Eight (62%) of the 13 had one or more relapses; of these, three died of disease at 35, 108, and 120 months, respectively, and another patient was alive with disease at 96 months. The other four patients who were treated after a relapse showed no evidence of disease after relapse at 36, 57, 63, and 146 months, respectively. Two of the 13 patients had tumor considered unresectable at the time of diagnosis; both died of disease at 5 and 10 months, respectively. Neither MI nor cytologic atypia were predictive of tumor recurrence or death from tumor. We also extracted clinical and morphologic data from all previous reports of primary extrauterine ESS, combined them with our 20 patients, and then compared the combined group with 17 cases of primary high-stage uterine ESS we presented in an earlier report. Not surprisingly, the behavior of the primary extrauterine ESS was more reminiscent of high-stage primary uterine ESS than low-stage primary uterine ESS.