Purpose : To determine the relationship between DNA damage and radiosensitivity at low doses (1-10Gy) for the initial DNA damage and residual DNA damage remaining after 12-h repair. Materials and methods : Eleven cell lines, normal human lung epithelial L132, HT29 human colon carcinoma, ATs4 human ataxia telangiectasia, normal CHO-K1 hamster, repair-deficient xrs1 and xrs5 mutants, repair-deficient SCID rodent cell line, the human normal fibroblast 1BR.3, human ataxia telangiectasia fibroblast AT1BR and the repair-deficient fibroblasts 180BR.B and 46BR.1 were irradiated with 60 Co γ-rays. Radiosensitivity was measured by clonogenic survival assay. DNA damage was measured by fluorometric analysis of DNA unwinding (FADU). Results : The radiosensitivity in the 11 cell lines ranged from SF2 of 0.02-0.61. By FADU assay, the undamaged DNA at 5-Gy ranged from 56 to 93%. The initial DNA damage and radiosensitivity were highly correlated (r 2 =0.81). After 5-Gy irradiation and 12-h repair, two groups of cell lines emerged. Group 1 restored undamaged DNA to a level ranging from 94 to 98%. Group 2 restored the undamaged DNA to a level ranging from 77 to 82%. No correlation was seen between residual DNA damage remaining after 12-h repair and radiosensitivity. Conclusion : It is shown that the initial DNA damage correlates with radiosensitivity at low doses of irradiation. This suggests that the initial DNA damage must be considered as a determinant for radiosensitivity.