We have reviewed the histopathology of lung cancer patients seen over the past 13 years at RPMI. Assessment of this data indicates that adenocarcinoma is becoming progressively more prevalent as related to the other forms of lung cancer. Factors which in part may account for this increased prevalence are: 1) changes in criteria for reading histopathology of lung cancer, particularly since 1967; 2) the increased incidence of lung cancer among the female population who have a propensity for adenocarcinoma; and 3) occupational and environmental factors. In 1974 adenocarcinoma for the first time became the most prevalent type of lung cancer at RPMI. Whatever the reason, if our data are truly representative of a national trend, adenocarcinoma will soon become the most prevalent type of lung cancer in the United States. This fact may result in an increasing death rate since the present 18-month survival rate for adenocarcinoma is substantially less than for squamous cell carcinoma, which has in the past been the prevalent form of the disease. As the smoking habits of women more closely approximate those of men, we expect that the incidence and mortality of lung cancer will prove to be quite similar in both sexes.