The annual incidence rates (crude and age-standardized) and numbers of new cases of 18 different cancers have been estimated for the year 1985 in 24 areas of the world. The total number of new cancer cases (excluding non-melanoma skin cancer) was 7.6 million, 52% of which occur in developing countries. The most common cancer in the world today is lung cancer, accounting for 17.6% of cancers of men worldwide, and 22% of cancers in men in the developed countries. Stomach cancer is now second in frequency (it was slightly more common than lung cancer in 1980) and breast cancer—by far the most important cancer of women (19.1% of the total)—is third. There are very large differences in the relative importance of the 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, mouth and pharynx, liver and oesophagus. The implications of these patterns for cancer control, and specifically prevention, are discussed. Tobacco smoking and chewing are almost certainly the major prevent able causes of cancer today.