The expression of fragile sites induced by aphidicolin (APC) was evaluated on metaphase chromosomes obtained from the peripheral blood lymphocytes of 26 women with breast cancer and 15 sex- and age-matched normal controls. Both the proportion of damaged cells (P < 0.001) and the mean number of gaps and breaks per cell (0.02 < P < 0.05) were significantly higher in the patient group. There were no differences in either the age-related fragile site levels or the expression of single fragile sites between patients and controls. Our findings indicate an increased genetic instability in women with breast carcinoma.