The clinicopathologic features of 21 cases of otherwise typical serous borderline tumors that contained small foci of stromal invasion were reviewed. The mean age of the patients was 43 years and six of them were pregnant at the time of diagnosis. Nineteen tumors were stage I, one was stage III (para-aortic lymph node involvement) and one was stage IV (parenchymal liver metastasis). The tumor invaded the stroma predominantly as individual cells or nests or clusters of cells with abundant eosinophilic cytoplasm (17 cases), as small confluent nests with a cribriform pattern (two cases) and as rounded aggregates of papillae (two cases). Seven women were treated with bilateral salpingo-oophorectomy and hysterectomy; 13 had less than bilateral oophorectomy. Of the 17 patients for whom followup data were available, 16 were without evidence of disease 1 to 11 (mean, 5.2) years postoperatively, and one patient had a serous borderline tumor with microinvasion in a conserved contralateral ovary 2.8 years postoperatively, but was well 6 months after a partial oophorectomy. These data suggest that serous borderline tumors with microinvasion have a prognosis similar to that of the usual serous borderline tumor, and that conservation of the contralateral ovary and uterus may be acceptable therapy in young women who wish to preserve their fertility.