Site-specific familial aggregation and evidence supporting Mendelian codominant inheritance have been shown in lung cancer. In characterizing lung cancer families, a number of other cancers have been observed. The current study evaluates whether first-degree relatives of early onset lung cancer cases are at increased risk of breast cancer. Families were identified through population-based lung cancer cases and controls under 40 years of age. Cases were ascertained through the Metropolitan Detroit SEER registry; controls through random-digit dialing. Data were available for 384 female relatives of 118 cases and 465 female relatives of 161 controls. Breast cancer in relatives was evaluated after adjusting for age, race, sex, and smoking status of each family member and the sex and age of the probands. A positive family history of early onset lung cancer increased breast cancer risk among first-degree relatives 5.1-fold (95% CI, 1.7-15.1). Relatives of cases with adenocarcinoma of the lung were at highest risk (RR = 6.3, 95% CI 2.0-20). Mean age of breast cancer diagnosis among relatives of cases was 52.2 years and not statistically different from relatives of controls. Three case families also reported early ovarian cancers (mean age of diagnosis of 35 years). These findings suggest that shared susceptibility genes may act to increase risk of early onset lung and breast cancer in families.