The role of hereditary factors in lung cancer is less well understood than in many other human neoplastic diseases. We used a nation-wide family dataset to search for evidence for a genetic predisposition in lung cancer. The Swedish Family-Cancer Database includes all Swedes born in 1932 and later (0- to 68-year-old offspring) with their parents, totaling over 10.2 million individuals. Cancer cases were retrieved from the Swedish Cancer Registry up to year 2000. Standardized incidence ratios (SIR) and 95% confidence limits (CI) were calculated for age-specific familial risks in offspring by parental or sibling proband, separately. A Kappa test was used to examine the association between familial risk and histology. Compared to the rate of lung cancers among persons without family history, a high risk by parental family history in adenocarcinoma (2.03) and large cell carcinoma (2.14) was found, and only a slightly lower risk was found among patients with squamous cell carcinoma (1.63) and small cell carcinoma (1.55). Among siblings, an increased risk was shown for concordant adenocarcinoma and small cell carcinoma at all ages and for all histological types when cancer was diagnosed before age 50. At young age, risks between siblings were higher than those between offspring and parents. The present data suggest that a large proportion of lung cancers before age 50 years appears to be heritable and probably due to a high-penetrant recessive gene or genes that predispose to tobacco carcinogens; however, this hypothesis needs to be tested in segregation analysis with a large number of pedigrees.