In order to assess the usefulness of chromosome aberrations in predicting breast cancer risk, 10 patients with breast cancer diagnosis and appropriately matching 10 healthy controls were chosen. Spontaneous and radiation induced unstable chromosome aberrations in peripheral blood lymphocytes were compared in the two groups. When the spontaneous aberration frequencies were compared, acentric chromosome frequency, scored in the group of patients was significantly higher than that found in the control group (p < 0.01). Absolute aberration frequencies as a determinant of radiosensitivity were calculated by subtracting spontaneous aberration frequencies from the frequencies that were obtained following 2 Gy of Co-60 gamma irradiation. Absolute dicentric chromosome frequency significantly increased in the patients' group (p < 0.01) as compared to that observed in the control group. Increases in either spontaneous acentric chromosome frequency or dicentric chromosome frequency as a determinant of an enhanced radiosensitivity in the group of patients may be valuable in predicting breast cancer risk. The studies involving unstable chromosome aberrations can be easily performed and can facilitate cancer diagnosis with minor effort and low cost.