Multiple primary cancers arise because of inherited or acquired deficiencies, and their causes may depend on the first primary cancer, or they may be entirely independent. We used a nation-wide family dataset to search for evidence for a genetic predisposition to lung cancer. The Swedish Family-Cancer Database includes all Swedes born in 1932 and later with their parents, totalling 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 first and second primary lung cancers by a family history. The incidence of second primary lung cancer was nine times higher among cases with familial lung cancer compare to that of first primary lung cancer. The proportion of multiple primary lung cancer patients with family history for lung cancer was 4.7% (9/190) for men and 6.5% (5/77) for women. Lung cancer patients with a family history of lung cancer were at a significantly increased risk for subsequent primary lung cancer among both men (SIR=9.89, 95%CI 4.48-18.66) and women (SIR=17.86, 95%CI 5.63-42.00). The corresponding SIRs in patients without a family history were 2.04 (95%CI 1.75-2.36) and 5.10 (95%CI 3.99-6.43) for men and women, respectively. The present study suggests that the development from the first primary lung cancer to the second primary lung cancer may be more strongly affected by genetic factor than the first primary lung cancer.