To overcome the limitations of existing biodosimetry methods, we examined dose- and time-dependent gene expression changes in human peripheral blood lymphocytes after exposure to low-, medium- and high-dose ionizing radiation and searched for genes suitable for predicting radiation doses in the low-dose range. Additionally, the experiments are intended to provide new insights into the biological effects of exposures to low-, medium- and high-dose radiation. Gene expression analysis using whole human genome DNA microarrays was performed in human blood from six healthy donors irradiated ex vivo with 0, 0.02, 0.1, 0.5, 1, 2 and 4 Gy (γ rays, ¹³⁷Cs) at 6, 24 and 48 h after high-dose exposure (0.5–4 Gy), and at 24 and 48 h after low-dose exposures of 0.02 or 0.1 Gy. DNA microarray-based alterations in gene expression were found in a wide dose range in vitro and allowed us to identify nine genes with which low radiation doses could be accurately predicted with a sensitivity of 95.6%. In the low-, medium- and high-dose range, expression alterations increased with increasing dose and time after exposure, and were assigned to different biological processes such as nucleosome assembly, apoptosis and DNA repair response. We conclude from our results that gene expression profiles are suitable for predicting low-dose radiation exposure in a rapid and reliable manner and that acute low-dose exposure, as low as 20 mGy, leads to well-defined physiological responses in human peripheral blood lymphocytes.