To test the contribution of homologous recombinational repair (HRR) in repairing DNA damage sites induced by high-energy iron ions, we used (1) HRR-deficient rodent cells carrying a deletion in the RAD51D gene and (2) syngeneic human cells impaired for HRR by RAD51D or RAD51 knockdown using RNA interference. We found that in response to exposure to iron ions, HRR contributed to cell survival in rodent cells and that HRR deficiency abrogated RAD51 focus formation. Complementation of the HRR defect by human RAD51D rescues both enhanced cytotoxicity and RAD51 focus formation. For human cells irradiated with iron ions, cell survival was decreased, and in p53 mutant cells, the levels of mutagenesis were increased when HRR was impaired. Human cells synchronized in S phase exhibited a more pronounced resistance to iron ions compared with cells in G₁ phase, and this increase in radioresistance was diminished by RAD51 knockdown. These results indicate a role for RAD51-mediated DNA repair (i.e. HRR) in removing a fraction of clustered lesions induced by charged-particle radiation. Our results are the first to directly show the requirement for an intact HRR pathway in human cells in ensuring DNA repair and cell survival after exposure to high-energy high-LET radiation.