The clinical and pathologic features of 2 malignant Brenner tumors (MBT) were compared with those of 13 primary ovarian transitional cell carcinomas (TCC). Though histologically both tumors were invasive transitional cell carcinoma, TCC differed from MBT by the absence of benign Brenner component and stronger aggressiveness in behavior. Among tumors of both types in similar clinical stage, most of TCC (6/9) died within 1 to 2 years after the operation, but one of MBT died 4 years postoperatively. The distinction between MBT and TCC bearing on therapeutic as well as prognostic implications is discussed.