Mammography using 26-30 kVp X rays is routinely used in breast cancer screening. Discussion about the radiation-related risk associated with this methodology is ongoing. For radioprotection purposes, a quality factor of 1 has been assigned for all photon energies. However, the relative biological effectiveness (RBE) could increase as the photon energy decreases. Analyzing different biological parameters, for 30 kVp X rays, RBE values from 1 to 8 have been estimated. In the present study, a cytogenetic FISH evaluation of the RBE of 30, 80 and 120 kVp X rays has been done. Blood samples were irradiated with 10 doses from 0.05 to 3 Gy for each energy studied. The yields of translocations and dicentrics were determined by fluorescence in situ hybridization (FISH) using whole chromosome probes for chromosomes 1, 4 and 11 together with a pancentromeric probe. The α coefficients of the dose-effect curves for dicentrics, minimum number of breaks needed to produce exchange-type aberrations, and apparently simple translocations were used to estimate the RBE. Using the curves obtained for 120 kVp as a reference, the RBE values for dicentrics were 1.08 ± 0.43 and 1.73 ± 0.59 for 80 and 30 kVp X rays, respectively; for minimum number of breaks these values were 1.38 ± 0.39 and 1.42 ± 0.41, and for apparently simple translocations they were 1.26 ± 0.40 and 1.51 ± 0.47, respectively. Moreover, the induction of complex aberrations by these energies was compared. The percentage of complex aberrations relative to total aberrations showed a significant tendency to increase as X-ray energy decreased: 7.8 ± 1.19, 9.8 ± 1.6 and 14.1 ± 1.9 respectively (P < 0.02). © 2008 by Radiation Research Society.