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A substantial proportion of women with breast cancer exhibit an abnormally high radiosensitivity as measured by the frequency of chromatid breaks induced in G[SUB2]-phase, PHA stimulated lymphocytes. Chromatid break frequencies were compared for a cohort of previously) untreated sporadic breast cancer patients and hospital outpatient controls. In the breast cancer group 46% showed high radiosensitivity compared to 14% of controls (P < 0.001). Comparison of those breast cancer patients with a high G[SUB2] radiosensitivity (G[SUB2]RS) versus those with a low G[SUB2]RS showed no difference in menopausal status or age but the high G[SUB2]RS group had on average a lower score on the Nottingham Prognostic Index. Predicted survival in the high G[SUB2]RS group at 15 years was 55% compared to 36% for the low G[SUB2]RS group. Furthermore, 81% of tumours from the high G[SUB2]RS were oestrogen receptor positive compared to 45% from the low G[SUB2]RS group. Thus high G[SUB2]RS identifies a sub-population of patients with distinctive tumour characteristics and with a predicted improved prognosis as compared with those in the low G[SUB2]RS group. Our findings imply that besides influencing risk of breast cancer the genetic factors determining G[SUB2]radiosensitivity also influence the tumour characteristics and prognosis in these patients. [ABSTRACT FROM AUTHOR]