The annual incidence rates (crude and age-standardized) and numbers of new cases of 25 different cancers have been estimated for the year 1990 in 23 areas of the world. The total number of new cancer cases (excluding non-melanoma skin cancer) was 8.1 million, just over half of which occur in the developing countries. The most common cancer in the world today is lung cancer, accounting for 18% of cancers of men worldwide, and 21% of cancers in men in the developed countries. Stomach cancer is second in frequency (almost 10% of all new cancers) and breast cancer, by far the most common cancer among women (21% of the total), is third. There are large differences in the relative frequency of different cancers by world area. The major cancers of developed countries (other than the 3 already named) are cancers of the colon-rectum and prostate, and in developing countries, cancers of the cervix uteri and esophagus. The implications of these patterns for cancer control, and specifically prevention, are discussed. Tobacco smoking and chewing are almost certainly the major preventable causes of cancer today.