Lung cancer is uncommon in individuals age 40 or less. We reviewed the Cancer Registry files of the Ben Taub General Hospital (Houston, TX) from 1971 to 1989 and identified 1678 patients with a documented diagnosis of lung cancer. Among these 1678 patients, 50 (2.98%) were age 40 or less. Thirty-five (70%) of the 50 patients were men and 15 (30%) were women. Their median age was 37 (range of 24 to 40). A smoking history was available in 37 patients. Thirty-five (94.5%) of the 37 patients who were smokers had a > 20-pack per year history of smoking. Four patients were intravenous drug abusers, and one of these four tested positive for the immunodeficiency virus. Twenty-seven (54%) had adenocarcinoma, eight (16%) had squamous cell carcinoma, and six (12%) had other nondescript, nonsmall-cell carcinoma types. In contrast, the proportion of adenocarcinoma for the (all-age) group of 1678 patients with lung cancer was 28.2%. This difference in the proportion of adenocarcinoma between the two age groups was statistically significant (Pearson's Chi 2, 13.7039, p < 0.0005). Thirty-one (77.5%) of the 50 patients had unresectable disease at diagnosis (12 had stage IIIb and 19 had stage IV). The median survival from diagnosis was 26 weeks. These findings suggest that (1) smoking is an important risk factor for this subset of young patients, (2) the proportion of adenocarcinoma is higher in the young compared with the entire group of lung cancer patients, which included patients of all ages, and (3) young patients tend to present with advanced disease at diagnosis, resulting in an extremely poor survival.