SUMMARY.-Three hundred and nineteen patients with primary adenocarcinoma of the ovary were studied to define those factors, many of them histopathological, which influence survival. The paper considers the stage of spread at the time of operation, the histological type of the tumour, its grade and in particular its mitotic activity, which proved a significant feature per se in assessing prognosis in ovarian cancer.A SURVEY has been carred out on 319 patients with primary adenocarcinoma of the ovary to evaluate some of the factors which influence survival in this disease. It became apparent during the investigation that the mitotic count was of considerable prognostic significance. Other factors considered were the histological type of the tumour, the stage to which it had spread at the time of operation and its histological grade. Ford (1928) was the first person to evaluate the prognosis according to the stage of spread at the time of operation, and the first large series was published in 1932 (Heyman, 1932). The first paper on grading appeqred at much the same time (Taylor, 1929), and most series since that of Pemberton (1940) have treated it as an important prognostic factor.Although cellular atypicality, as a feature of malignancy, has been discussed frequently in relation to grading, mitotic activity has received much less attention. Barzilai (1943) reported that mitoses were "frequently seen " in the group of tumours which she called " sero-anaplastic", and Allan and Hertig (1949) used profusion of mitoses as one of the indicators of the most malignant neoplasms. But at the time when this study was undertaken, no publication had been found in which mitoses were considered as a separate feature. The only published work specifically devoted to this point was by Novak and Woodruff (1967) after the present investigation had been largely completed. It confirms that the count of mitoses per high power field is of prognostic significance in carcinoma of the ovary.