The magnitude of the cancer problem, as conveyed by the incidence, mortality and prevalence figures, was studied by analysis of the material of the Finnish Cancer Registry from 1953-1970. The prevalence was defined as the number of persons who had cancer, or had ever had cancer, and were living on December 31, 1970. The incidence and mortality of cancer in males exceeded those in respect of females, whereas the prevalence with females was clearly higher than that with males. According to all 3 indices, the commonest site of cancer was in the lungs in males. In females, cancer of the breast was the commonest malignant neoplasm measured by incidence and prevalence, whereas in mortality statistics it was second to cancer of the stomach. Cancers with poor prognosis, such as cancer of the lung, stomach, pancreas and esophagus, and leukemia, exhibited relatively low prevalence figures, whereas cancers with good prognosis, such as cancer of the breast, uterus, larynx and lip, showed relatively large numbers of prevalent cases. In females, cancer of the genitals, including the breasts, constituted nearly as many prevalent cases as did the total figure in males. As the 3 cancer indices measure different things, the nature of the index used should be taken into account in evaluation of the magnitude of the cancer problem.