Tobacco smoking is accepted as a major cause of cancers of the lung, larynx, oral cavity and pharynx, oesophagus, pancreas, kidney and bladder. The proportions of these cancers that are due to smoking were estimated for the year 1985 for 24 areas of the world. Fifteen percent--1.1 million new cases per year--of all cancer cases are attributed to cigarette smoking, 25% in men and 4% in women. In developed countries, the tobacco burden is estimated at 16% of all annual incident cases. In developing countries, the corresponding figure is 10%. In total, 85% of the 676,000 cases of lung cancer in men are attributable to tobacco smoking. The highest attributable fractions (AF: 90-93%) are estimated in areas where the habit of cigarette smoking in men has been longest established: North America, Europe, Australia/New Zealand and the former USSR. Among the other 6 cancer sites considered in this analysis, those with the largest fractions of tobacco-related cases are the larynx, mouth and pharynx (excluding nasopharynx) and oesophagus. In regions where males have smoked for several decades, 30 to 40% of all cancers in this sex are attributable to tobacco. Unless tobacco-control efforts in developing countries are strengthened, the massive rise in cigarette consumption over the last few decades will produce a comparable rise in cancer in these countries within the next 20 to 30 years.