Microscopic, heterotopic extraovarian sex cord-stromal proliferations have only recently been reported in the literature. We describe the largest series to date, of 30 cases of microscopic, incidentally detected, heterotopic extraovarian sex cord-stromal proliferation, in women aged 25-79 yr who had undergone surgery for a range of benign and malignant gynecologic conditions. In 14 patients the foci of proliferation comprised ovarian cortical stroma, in some cases with an ovarian fibroma-like appearance. Ten cases of adenofibroma and cystadenofibroma were also identified, including 1 Brenner adenofibroma; 2 cases comprised both ovarian cortical stroma and serous cystadenofibroma; 4 cases showed sex cord proliferation resembling microscopic adult granulosa cell tumors. Immunohistochemistry, where possible, confirmed the sex cord nature of the heterotopic proliferations. The foci of proliferation were <1-7 mm, and most were at the fimbrial end of the fallopian tube. These proliferations are likely to be encountered with increasing frequency as we sample the adnexa more extensively. Previous reports postulated that the proliferations probably represent embryonic rests caused by anomalous migration but we suggest that incorporation of exposed ovarian parenchymal tissue into the fimbrial stroma at the time of ovulation may be another possible cause.