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Background: Acute lymphoblastic leukemia (ALL) constitutes a heterogeneous group of diseases characterized by abnormal proliferation and accumulation of immature lym- phoblasts arrested at various differentiation stages. Increasing evidence suggests that chromosomal defects are present in these patients. The aim of this study is to investigate the chromosomal radiosensitivity in a group of ALL patients. Materials and Methods: To analyze chromosomal radiosensitivity of ALL patients, lymphocytes of 20 patients were cultured followed by exposure to γ-ray irradiation to detect the chromosomal aberrations as an indicator of radiosensitivity. Cells were scored for the number of aberrations (chromatid breaks, chromatid gaps, chromosome breaks, chromosome gaps and chromatid exchanges). Results were compared with healthy individuals, and ataxia telangiectasia (AT) patients as positive control. Results: On average number of aberrations in ALL patients was significantly higher than that in healthy controls. We found 65% of ALL patients appeared to be susceptible to in vitro irradiation. Chromosomal radiosensitivity of 35% patients was not different from healthy donors. Ataxia telangiectasia patients showed the highest degree of lymphocyte radiosensitivity. The results also indicate that there is a good correlation between the two assays, G2 and G0, using the same blood sample for both assays. Conclusion: According to the result, we concluded that most of the ALL patients are sensitive to ionizing radiation and therefore should be protected from unnecessary diagnostic and therapeutic procedures using ionizing radiation. [ABSTRACT FROM AUTHOR]