Borderline tumors with papillae that are architecturally similar to those of serous tumors but with a lining of more than one mullerian cell type have not been well characterized in in the literature. We have studied 36 such tumors. The patients averaged 35 years of age. Twenty-two percent of the tumors were bilateral; all were confined to the ovaries as confirmed at operation. Fifty-three percent were associated with endometriosis. Follow-up information was available on 34 patients for a mean interval of 4.8 years. A tumor developed in the contralateral ovary in one patient 2 years after unilateral salpingo-oophorectomy. Three patients had pelvic recurrences between 7 months and 3 years, but all of them were successfully treated and none have died. These tumors differ clinically and pathologically from intestinal-type mucinous borderline tumors, but they have many similarities with mullerian mucinous borderline tumors and, to a lesser extent, with serous borderline tumors.