The role of hereditary factors in tumor development has been less well understood for lung cancer than for many other human neoplastic diseases. The nation-wide Swedish Family-Cancer Database was used on 10.2 million individuals and 4524 lung cancers to calculate standardized incidence ratios (SIRs) and 95% confidence intervals (CIs) for histological subtypes of lung cancer in 0-66-year-old offspring by cancers in family members. Additionally, SIRs for second lung cancers were analyzed. SIRs in offspring for all lung cancer were increased to 1.87 (95% CI 1.66-2.10), adenocarcinoma to 2.15 (1.77-2.59) and squamous cell carcinoma to 1.86 (1.39-2.44) when a parent presented with lung cancer. The familial risk was not dependent on diagnostic age. Lung cancer associated with parental rectal, cervical, kidney, urinary bladder and endocrine gland cancer. The population attributable fraction of familial lung cancer was 2.97%. Risks for second lung cancers were increased in men and women after smoking and life style related sites, and after skin cancer, non-Hodgkin's lymphoma and Hodgkin's disease.