Benign endometrial polyps belong in the differential diagnosis of adenofibroma and adenosarcoma. There is, however, little information about the range of stromal mitotic activity, stromal cellularity, and stromal atypia in benign endometrial polyps, rendering the differential diagnosis with the aforementioned tumors problematic. In this study, the stroma of 66 polyps 1 cm or more in greatest dimension from 56 patients was analyzed for stromal mitotic activity, cellularity, and atypia. Sixteen (24%) had an almost completely fibrotic stroma that had rare mitoses, little cellularity, and no atypia. However, 50 polyps (76%) had stroma that was predominantly endometrial or was a mixture of endometrial-type stroma and fibrous stroma. In these polyps stromal mitoses were relatively common, averaging 1.2/10 MFs/HPFs (range, 0-5.8 MFs/10 HPFs). Stromal cellularity was frequently equal to or mildly increased over adjacent nonpolypoid endometrial stroma and mild nuclear atypia (enlarged stromal nuclei) was also common. Twelve polyps (24%) from the group of 50 had two or more MFs/10 HPFs, a mitotic rate present in some adenosarcomas. None of these polyps had other features necessary for the diagnosis of adenofibroma or adenosarcoma and follow-up in all patients was uneventful (average follow-up, 96 months). It is concluded that benign polyps that retain areas of endometrial-type stroma often have mitotic activity and that significant stromal mitotic activity (> or = 2 MFs/HPFs) is relatively common. These polyps do not have significant stromal atypia nor do they have a marked increase in stromal cellularity. Thus, in the absence of other supportive features, stromal mitotic activity alone should not be regarded as a worrisome finding.