Primary peritoneal serous micropapillomatosis of low malignant potential, or serous borderline tumor of the peritoneum, is a relatively rare lesion that is histologically indistinguishable from peritoneal "implants" associated with ovarian papillary serous tumors of low malignant potential. We analyzed 17 cases to further define the pathologic features and prognosis of this entity. The ages of the patients ranged from 16 to 67 years (mean, 33 years). Eight patients were symptomatic with chronic pelvic or abdominal pain (five patients), adnexal mass (one patient), small-bowel obstruction (one patient), and possible endometriosis (one patient). In nine cases (53%), peritoneal serous micropapillomatosis of low malignant potential was an incidental finding discovered during evaluation or treatment of other conditions. Grossly, the peritoneal lesions were focal or diffuse. They commonly appeared as miliary granules and often were believed to be peritoneal carcinomatosis. Microscopically, peritoneal serous micropapillomatosis of low malignant potential had all of the patterns seen in superficial ("noninvasive") peritoneal implants of ovarian serous borderline tumors. Psammoma bodies were a prominent feature of all cases. Twelve patients also had typical endosalpingiosis. Most patients were treated by hysterectomy and bilateral salpingo-oophorectomy. Surgical treatment in seven patients consisted only of biopsy. Ten patients had residual unresected disease at the time of their initial operation. Several patients received adjuvant chemotherapy. Follow-up was available for 14 of the 17 patients. One patient died of metastatic breast carcinoma at 3.8 years; another patient died 7 weeks after operation, possibly as a complication of therapy. The other 12 patients were alive at last known contact after follow-up intervals of 8 months to 16.2 years (mean, 7.5 years). Two of these 12 patients developed multiple episodes of small-bowel obstruction due to persistent peritoneal serous micropapillomatosis of low malignant potential; neither received adjuvant chemo- or radiotherapy. Both were alive without progressive disease 10.9 and 16.2 years after initial diagnosis, respectively. This excellent prognosis supports a regimen of conservative therapy for these patients.