Despite improved modern diagnostic techniques, many patients with primary lung cancer escape detection of their disease during life. In a review of postmortem records at a university hospital, 28 percent of 153 primary lung cancers found at necropsy had not been diagnosed while the patient was alive. The male/female ratio was 1.3 in this undetected group, compared with 2.3 in the detected group. The main clinical features that seemed to lead to nondiagnosis were a terminal clinical state in patients who were too sick for further diagnostic searches, the absence of suggestive primary symptoms, a chest x-ray film interpreted as not showing primary lung cancer, and the absence of cigarette smoking. Among the patients with lung cancer at necropsy, the proportion of nonsmokers was higher in the previously undiagnosed group than in the group with antemortem diagnoses, even when patients were stratified for primary symptoms. The findings suggest the need for diagnostic alertness to the possibility that curable lung cancer can occur in patients who have a positive chest-film lesion but who are nonsmokers and who lack typical symptoms.