This paper describes the relative frequencies and changes in the morphologic patterns of lung cancer, as documented in the histologic and cytologic specimens from 2,580 patients diagnosed and treated at Duke University Medical Center over a period of 15 consecutive years. During the first five years, the relative frequencies of the various types of lung cancers were, in descending order, squamous cell carcinoma (43.1%), large cell undifferentiated carcinoma (22.5%), adenocarcinoma of the acinar type (18.6%), small cell undifferentiated carcinoma (11.6%), bronchioloalveolar carcinoma (3.5%) and adenosquamous carcinoma (0.7%). During the second and third five-year periods, squamous cell carcinoma remained the most common neoplasm, but declined to 35.7%, while adenocarcinoma of the acinar type became the second most common lung cancer at 22.0%. The absolute and relative incidences of lung cancer in women showed a striking increase from 21.8% to 29.9%. During this same period, adenocarcinoma of the acinar type replaced squamous cell carcinoma as the most common primary lung cancer in women.