In this study, we aimed to confirm the finding that chromosome 5 aberrations are predisposing factors for lung cancer. The study population consisted of 118 previously untreated lung cancer patients and 101 healthy controls. Lymphocytes were treated with bleomycin for 5 hr and then allowed to recover in a drug-free medium for 48 hr. The mean number of cells with chromosome 5 abnormalities among 100 cells examined was significantly higher in patients (9.12) than in controls (4.69) (p < 0.001). The most frequent aberration was a 5q deletion and the breakpoints clustered at the 5q13-5q31 region. We then dichotomized the number of induced chromosome 5 abnormalities in peripheral blood lymphocytes by the 75th percentile in that of the controls. 103 (87.3%), of the 118 patients, but only 31 (30.7%) of the 101 controls, exhibited induced breaks above this point. After adjustment for age, sex, ethnicity and smoking status, we found that the sensitive group was at 14.4-fold increased risk for lung cancer. There was also a significant (p < 0.01) gradient of increased risk for lung cancer with an increasing number of chromosome 5 lesions. Therefore, chromosome 5 lesions, especially those at 5q, may be a molecular target of carcinogens in the development of lung cancer.