A clinico-pathological study of 48 malignant Brenner tumors (MBT) includes three new cases and 45 cases from the literature since 1945. Construction of a profile of this neoplasm was undertaken. The mean age of patients was 60 years, and most were postmenopausal with abdominal symptoms and a unilateral abdominopelvic mass. Only one case had bilateral tumors. The pattern of spread of this tumor is usually regional with only occasional distant metastases. Diffuse peritoneal seeding is not a characteristic feature of MBT. 56% of the patients with MBT died of their tumor, usually within 3 years of initial diagnosis. Mean survival in fatal cases was 1 year. Microscopically, foci of characteristic benign Brenner tumor may or may not be seen in individual examples of MBT. Admixture of adenocarcinoma (often mucinous carcinoma) and squamous carcinoma may also be seen. diagnostic problems can arise when a mixture of histologic patterns is present. In order to establish the diagnosis of malignant Brenner tumor a prominent histologic pattern of malignant urothelium is usually necessary.