Reports of 78 cases of mucinous cystadenocarcinoma of the ovary were collected from the Emil Novak Ovarian Tumor Registry and the files of the Gynecologic Pathology Laboratory of the Johns Hopkins Hospital between the years 1942 and 1966. Two-year and 5-year followups were available for 91 and 83% of the cases, respectively. The prognosis was related to the most aggressive area of the tumor as the histologic grade was based on the maximum number of mitoses per high-powered field. In view of the clinical and histopathologic differences between these lesions and the serous and endometrioid varieties, it is suggested that each tumor be considered on its own merits in order to offer an accurate prognosis.