Trends in the histologic types of lung cancers during 1966-1985 by 5-year intervals were analyzed by using 4,419 cases in the National Cancer Center Hospital, Japan. Histologically confirmed cases increased to 76.6% in the last 5-year interval. The proportions of histological types in males have remained relatively unchanged; about 40% squamous cell carcinoma, one-third adenocarcinoma, and about 10% each of large cell carcinoma, small cell carcinoma and other types. In females, an increase of adenocarcinoma from 52% to 69.1% and a decrease of squamous cell carcinoma from 21% to 11.4% were noteworthy, and the other three types amounted to less than 10% each. These trends were compared with those obtained from the Annals of Pathological Autopsy Records in Japan during 1974-1983, which contained 26,844 lung cancer cases; the results were similar except for an increase of small cell carcinoma. The recent increase of small cell carcinoma was considered to be due to a change in diagnostic categorization, associated with a decreased rate of undifferentiated carcinoma. However, the increase of small cell carcinoma as well as adenocarcinoma in younger patients requires further study from both demographic and etiological viewpoints.