The alkaline comet assay was used to study the genomic instability of lymphocytes derived from untreated sporadic breast cancer patients (50 cases), and also following their in vitro irradiation up to 5 Gy. We compared the results (mean tail moment (MTM)) with a control population of 25 patients and with breast cancer patients who had been 'cured' of their disease, with a follow-up of 10 years or more (25 cases). At the basal level, 77.5% (P<0.01) of the untreated patients and 73.7% (P<0.05) of the 'cured' women had values higher than the basal cut-off level of 5.3, compared with only 44% of the controls. After in vitro irradiation, 83% of the untreated patients were above the cut-off value of 10.8 at the 5-Gy dose compared with only 48% of the controls (P<0.01). These results support the hypothesis that women affected by sporadic breast cancer have a constitutional genomic instability. The assessment of the prognostic value of this test could be of interest, particularly in women without axillary nodal involvement.