The correct diagnosis had not been made during life in 26% of 153 patients with lung cancer found in necropsies performed between 1971 and 1982. The likelihood of a correct antemortem diagnosis showed distinctive gradients in relation to the patients' history and amount of cigarette smoking, symptomatic manifestations, and anatomic extensiveness of the cancers. However, cigarette smoking still exerted a diagnostic effect in patients with similar symptoms and similar degrees of anatomic spread. Furthermore, if a lesion was present, chest films were more likely to be radiologically interpreted as a cancer in smokers. The results suggest that smokers receive preferential consideration regarding the diagnosis of lung cancer. This detection bias can have adverse scientific consequences in depriving nonsmokers of suitable therapy, in leading to falsely high estimates of the true magnitude of the smoking/lung cancer association, and in distracting etiologic attention from other agents that may cause lung cancer.