Severe acute toxicity to radiotherapy (radiohypersensitivity) can limit the effective use of radiotherapy and it is not usually possible to identify such individuals before treatment commences. Five patients with acute radiohypersensitivity (RH) were detected over a 3-year period. All five RH subjects demonstrated a significantly higher degree of radiation-induced chromosomal aberrations (ICA) in fresh blood lymphocytes when compared to normal controls. Results indicate the feasibility of using the ICA assay in conjunction with other tests to screen for radiosensitivity.