This study assessed the accuracy of obtaining smoking history, relationships between smoking and the histologic subtypes of lung cancer, past and present smoking history, and co-carcinogen history in 100 patients seen between 1982 and 1989. A standard questionnaire filled out by the patients, a data base filled out by the physician, and medical records were abstracted, and detailed information on smoking and co-carcinogen history was obtained. Eleven percent of the patients were nonsmokers and another 41 percent were former smokers who had quit smoking more than one year prior to the diagnosis of lung cancer. Mean ages at onset and cessation of smoking and diagnosis were 17, 59, and 62 years, respectively. The histologic subtypes were as follows: adenocarcinoma, 34; squamous, 18; small cell, 24; adenosquamous, nine; large cell, nine; and bronchioloalveolar carcinoma, six. Mean pack-years of cigarette smoking for the subtypes were as follows: squamous, 82; small cell, 78; large cell, 72; adenocarcinoma, 65; adenosquamous, 48; and bronchioloalveolar carcinoma, 41. The patient and physician questionnaires had comparable data on smoking status in continued smokers and never smokers. Many former smokers filled out the patient questionnaire as a nonsmoker, but on query by the physician admitted to smoking in the past. The physician data set was more accurate in former smokers than questionnaires completed by the patients. Patients with squamous and small cell carcinomas were heavier smokers than patients with adenosquamous and bronchioloalveolar carcinomas. About 50 percent were active smokers until the diagnosis of lung cancer, but only 18 percent of patients continued to smoke after the diagnosis. About 10 percent were never smokers and about 40 percent were former smokers. Most former smokers quit smoking less than five years antecedent to the diagnosis of lung cancer.