We have encountered 16 ovarian neoplasms of probable stromal origin whose most distinctive feature is microcystic change, which is usually conspicuous. On the basis of our extensive experience with ovarian tumors, the neoplasm is unique and warrants separate categorization; we have elected to designate it "microcystic stromal tumor" because of its most striking feature. The patients ranged from 26 to 63 (mean 45) years of age and typically presented with a pelvic mass. Hormonal manifestations were possibly present in only 2. All tumors were unilateral with a mean size of 8.7 (range: 2 to 27) cm and none had evidence of extraovarian spread. The tumors were solid-cystic (11 cases), solid (3 cases), or predominantly cystic (2 cases). The solid component was usually firm and tan or white-tan, but in 1 case was yellowish; soft foci were present in 3 cases and small foci of hemorrhage, necrosis, or both, in 3. On microscopic examination the appearance of the tumors varied according to the relative prominence of their 3 fundamental components: microcysts, solid cellular regions, and fibrous stroma. Microcysts dominated in 9 cases, were roughly equal to noncystic morphology in 5 cases and were minor in 2. The microcystic pattern was characterized by small rounded to oval cystic spaces, in areas coalescing to larger irregular channels; intracytoplasmic vacuoles were also frequently present. The solid cellular areas were usually focally intersected by fibrous bands and hyaline plaques reminiscent of thecoma. The cells contained moderately conspicuous finely granular, lightly eosinophilic cytoplasm, with generally bland, round to oval or spindle-shaped nuclei with fine chromatin and small indistinct nucleoli. Foci of bizarre nuclei were, however, present in 10 cases. Mitotic rate was low in all cases, ranging from 0 to 2 mitoses/10 high-power fields. Immunohistochemical results were as follows: CD10, 16/16 cases positive; vimentin, 16/16 cases positive; inhibin, 1/16 cases weakly positive; calretinin, 1/16 cases positive; cytokeratin, 4/16 cases focally positive; and epithelial membrane antigen, 0/16 cases positive. Microcystic change can be observed in a wide variety of ovarian tumors and the broad potential differential diagnosis is discussed in the text. For tumors which have been well sampled and exhibit (1) a microcystic pattern and regions with lobulated cellular masses with intervening, sometimes hyalinized fibrous stroma, (2) an absence of morphologic features enabling any other specific diagnosis in the sex cord-stromal category, (3) an absence of epithelial elements, and (4) an absence of teratomatous or other germ cell elements, we propose the designation "microcystic stromal tumor." The characteristic immunophenotype is CD10/vimentin+/epithelial membrane antigen-, with focal cytokeratin-positivity in one-quarter of cases; inhibin and/or calretinin are usually negative. Seven patients with available follow-up are without evidence of disease at a mean of 4.25 years (range: 1.5 to 12.5 y) from the time of initial diagnosis. These tumors, to date, have occurred over a wide age range in postpubertal females, are characteristically unilateral, and confined to the ovary at presentation. They represent, in addition to the sclerosing stromal tumor (segregated out 3 decades ago), a distinctive subtype of ovarian tumor, likely also belonging to the stromal category based on current evidence.