A review of the histopathology and past history of 2229 patients with primary lung cancer diagnosed at the Yonsei University Medical Center from 1981 to 1990 was performed to investigate the changes in histologic types and the relationship to smoking history. The most frequent histologic type of lung cancer was squamous cell carcinoma (956 patients, 54.0%) followed by adenocarcinoma (311 patients, 17.6%) in males (1772 patients), and adenocarcinoma (206 patients, 45.1%) followed by squamous cell carcinoma (126 patients, 27.6%) in females (457 patients). In both sexes, the predominant type was adenocarcinoma under the age of 40, whereas squamous cell carcinoma was the most frequent type above the age of 40. While squamous cell carcinoma decreased over 10 years (54.3% in 1981, 44.3% in 1990), adenocarcinoma showed a gradually increased incidence (17.0% in 1981, 28.3% in 1990) in both sexes, and the proportion of small cell carcinoma and large cell carcinoma remained unchanged. These changes in histologic type were more prominent in non-smokers. In conclusion, the increasing incidence of adenocarcinoma in both sexes, especially in non-smokers, suggests the possible presence of etiologic factors other than smoking, such as environmental pollution.