Administration of ionizing radiation for diagnostic purposes can be associated with a risk for the induction of tumors. Therefore, particularly with regard to general screening programs, e.g. with mammography, cost-benefit considerations must be discussed including risk estimation depending upon the radiation quality administered. The present study was initiated to investigate the in vitro X-ray energy dependence for the induction of chromosomal aberrations in the two mammary epithelial cell lines, 184A1 and MCF-12A. The induced excess fragments, dicentric chromosomes and centric rings were analyzed and the relative biological effectiveness (RBE) was determined for 10 and 25 kV X-rays relative to 200 kV X-rays. The assumed energy dependence with higher values for 10 kV X-rays was confirmed for the excess fragments, with RBE _M values of 1.92 ± 0.26 and 1.40 ± 0.12 for 10 kV X-rays and 1.17 ± 0.12 and 0.97 ± 0.10 for 25 kV photons determined for cell lines 184A1 and MCF-12A, respectively. Meaningful results for the induction of dicentric chromosomes and centric rings were obtained only for higher doses with RBE values of 1.31 ± 0.21 and 1.70 ± 0.29 for 184A1 and 1.08 ± 0.08 and 1.43 ± 0.12 for MCF-12A irradiated with 25 and 10 kV X-rays, respectively.