A review and reclassification of all cases of bronchogenic carcinoma in the entire adult autopsy population, from 1912 to 1956, at the Presbyterian Hospital in New York revealed, in both sexes, a progressive change in the relative frequency of squamous-cell and undifferentiated carcinoma compared to adenocarcinoma. In males and females the autopsy incidence of adenocarcinoma has remained relatively constant over this 45-year period. In the male the ratio of squamous-cell and undifferentiated carcinoma compared to that of adenocarcinoma shifted progressively from a 0.66–1.0 ratio to a 3.75-1.0 ratio; in the female this has changed from a 0–1.0 ratio to a 1.5-1.0 ratio. In both sexes there also appears to have been an absolute increase in squamous-cell and undifferentiated carcinoma. This increase has been much more pronounced in the male. The shift in this ratio also appears to have begun 10 to 20 years later in the female than in the male.