In a study of 505 cases of lung carcinoma from a period in which there was a significant change in tobacco-smoking habits, a dramatic shift in the histologic type and location of the lung tumors was observed. Peripheral tumors, found in 30.7% of the carcinomas occurring before 1978, were found in 42% of the carcinomas from 1986 to 1989. The corresponding decrease in the centrally originating bronchial carcinoma was from 69.3% to 57.3%. The greatest change in histologic cancer type was that the incidence of bronchioloalveolar carcinoma more than doubled from 9.3% in the earlier period to 20.3% percent in the 1986-to-1989 period. Corresponding to the decreasing incidence of lung carcinoma, there is a decrease in cancers related to cigarette smoking. A study of cases of lung carcinoma among nonsmokers and former smokers showed a decreased incidence of the bronchogenic cancers and an increase of cancer occurring in the peripheral lung parenchyma. This finding should be validated in other population-based studies, and if confirmed, new studies should be undertaken in an attempt to discover the factors that play a role in the development of such cancers. As an example, viral oncogenes may be a possibility. Viruses were suggested in the past as being related to the development of some of these tumor types.