The frequencies of sister chromatid exchanges (SCE) in the peripheral lymphocytes of patients with lung cancer before and after initial chemotherapy were serially measured and the correlation between SCE frequencies and bone marrow toxicity was evaluated. The addition of mitomycin C to vindesine plus cisplatin increased SCE frequencies significantly in patients with non-small cell lung cancer. A significant increase in frequencies of SCE was observed in patients treated with cyclophosphamide plus adriamycin plus vincristine as compared with those treated with cisplatin plus etoposide in small cell lung cancer. A significant inverse correlation was observed between SCE frequencies in the peripheral lymphocytes 7 days after treatment (x) and the nadir value/pretreatment value of platelets (y)(r = -0.685, P = 0.0007, y = 0.842-0.022x). The relation between SCE frequency and leukopenia showed the same trend as thrombocytopenia, but the correlation was not statistically significant (r = -0.444, P = 0.057). SCE assay may have potential clinical use for the prediction of chemotherapy-induced thrombocytopenia.