Risk factors for early onset of lung cancer are relatively unknown. In a case-control study, carried out in Germany between 1990 and 1996, the effects of smoking and familial aggregation of cancer were compared in 251 young cases and 280 young controls (< or = 45 years) and in 2,009 older cases and 2,039 older controls (55-69 years). The male/female ratio was 2.6/1 in young patients and 5.6/1 in older patients. Adenocarcinomas were more frequent in young men than in older men (41 % vs. 28%). Duration of smoking and amount smoked showed significantly increased odds ratios for lung cancer in both age groups. Lung cancer in a first degree relative was associated with a 2.6-fold (95% confidence interval (CI) 1.1-6.0) increase in the risk of lung cancer in the young age group, but no elevated risk was seen in the older group (OR = 1.2, 95% CI 0.9-1.6). Smoking-related cancer in relatives with the age at diagnosis under 46 years was associated with an increased risk of lung cancer in the young group (OR = 5.6, 95% CI 0.7-46.9) but not in the older group (OR = 0.7, 95% CI 0.3-1.5). Results indicated that lung cancer risk in young and older age groups shows remarkable differences with respect to sex, histologic type, and genetic predisposition.