The numbers of new cancer cases in 16 common sites occurring in 1980 have been estimated for 24 areas of the world for which the United Nations produces population estimates. For the world as a whole, the total number of new cases was 6.35 million, almost exactly evenly divided between the developed and developing countries. In males, the most important sites were lung, stomach, colon/rectum, mouth/pharynx, prostate and oesophagus, and in females breast, cervix, colon/rectum, stomach, corpus uteri and lung. When the two sexes are combined, stomach cancer emerges as the most common cancer in 1980 (669,400 new cases per year), but this estimate is only slightly greater than that of lung cancer (660,500 new cases), and comparisons with earlier estimates for 1975 suggest that, with declining incidence rates for stomach cancer and the continuing rise for lung cancer, the latter would become the most common cancer in the world by the end of 1981. The implications for cancer control in the developed and developing countries of the world are discussed.