We used a case-control study design to determine the association between bleomycin-induced chromatid breaks and the risk of lung cancer in general and by specific histopathological types. Lymphocytes from primary blood cultures of 78 controls and 75 cases with 4 histopathological types of lung cancer were treated with 0.03 unit/ml bleomycin for 5 h, and the frequency of induced chromatid breakage and the locations of the breaks were determined in Q-banded preparations. After adjustment for their length, the larger chromosomes had more breaks than the smaller chromosomes in both cases and controls. The cases had significantly more breaks on chromosomes 4 and 5 than the controls did, with odds ratios (ORs) of 4.9 [95% confidence limits (CL), 2.0, 11.7] and 3.9 (95% CL, 1.6, 9.3), respectively. When the lung cancers were classified by histopathological type, adenocarcinomas had significantly more breaks on chromosomes 4 and 5, with ORs of 3.0 (95% CL, 1.0, 8.7) and 3.5 (95% CL, 1.2, 10.7), respectively. For squamous cell carcinoma, the ORs were significantly elevated for breaks on chromosomes 2, 4, and 5 with ORs of 3.5 (95% CL, 1.0, 11.7), 10.2 (95% CL, 2.5, 41.9), and 7.9 (95% CL, 1.9, 32.8). For small cell carcinoma, breaks on chromosomes 2 and 4 showed significantly increased ORs of 33.2 (95% CL, 2.2, 513.3) and 20.4 (95% CL, 1.7, 250.1), respectively. However, no specific chromatid breaks were detected in cases with large cell carcinoma. When the frequency of chromatid breaks at specific regions was calculated, breaks at 4p14, 4q27, 4q31, 5q21-q22, 5q31, and 5q33 were significantly more common in lung cancer cases than in controls. Lung cancer risk had a dose-response relationship with breaks on chromosomes 4 and 5. Cigarette smoking had a strong interaction with breaks on chromosomes 2, 4, and 5. The findings suggest that the susceptibility of particular chromosome loci to mutagenic damage may be a risk factor for specific types of lung cancer.