In the United States, lung cancer incidence and mortality rates are particularly high among African American men. Lung cancer tends to be most common in developed countries, particularly in North America and Europe, and less common in developing countries, particularly in Africa and South America. Over 85% of all lung cancers are attributed to cigarette smoking; however, only a fraction of long-term cigarette smokers develop lung cancer. Cancer of the lung is occasionally observed among nonsmokers, indicating a genetic susceptibility in lung tumorigenesis. The incidence of lung cancer seems to have increased in dogs as well as in humans and cigarette smoking can hardly be incriminated.