Fifty-six Brenner tumors of 51 patients were analyzed. The incidence of bilaterality was 9.8%. Most tumors were smaller than 2 cm, and constituted incidental finding. Most of the patients were 30-49 years old (mean 49.7, main 44). A high incidence of associated malignant neoplasms was noticed among the patients with Brenner tumor but was considered coincidental. No evidence of hormone secretion by the tumor was found. Out of 31 tumors, 27 occupied the cortex. Urothelial metaplasia of the ovarian surface epithelium was demonstrated in one case, and continuation of the Brenner column with the surface of ovary in another. There were 12 cases of mixed Brenner tumor in this series (21.1%). The most frequent associated constituents of the mixed Brenner tumors are mucinous cystadenomas. These tumors represent a single mixed neoplasm originating in a multipotential celomic cell which proliferates and differentiates into several mullerian forms. The histogenesis of medullary and hilar Brenner tumors can be explained on the basis of celomic metaplasia. Uncertainty persists in explaining the histogenesis of Brenner tumors mixed with mature cystic teratoma.