Abstract. 2-Deoxy-D-glucose (2DG), a well-known inhibitor of anaerobic glycolysis, is expected to exert cytotoxic and radiosensitizing effects. In order to test this hypothesis, the response of four tumor cell lines (U87-MG, GaMG, A549 and HT1080) to 2DG was analyzed for cell proliferation, changes in cell volume and nucleus size, as well as for radiation-induced DNA fragmentation, measured by the alkaline Comet assay. Two methods were used for loading cells with 2DG. The long-term method included cell cultivation in the presence of 5 mM 2DG for 24 h, while rapid intracellular delivery of 2DG was achieved by exposing the cells for 20 min to a hypotonic solution containing 100 mM 2DG. Irrespective of the loading method, 2DG inhibited the growth of HT1080 and A549 cells. In contrast, two glioblastoma lines (U87 and GaMG) were resistant to 2DG. In three of the four cell lines (all except HT1080), long-term treatment with 2DG reduced radiation-induced DNA fragmentation in conjunction with 2DG-mediated nucleus shrinkage (probably via chromatin condensation) in non-irradiated cells. Complementary volumetric experiments revealed the avid hypotonic uptake of 2DG by all tumor lines. Nonetheless, only HT1080 cells exhibited a significant increase in radiation-induced DNA fragmentation upon hypotonic loading with 2DG, associated with marked nucleus expansion in non-irradiated samples. Our data suggest that, dependant on cell type as well as on medium composition and tonicity, sugar treatment can induce the compaction or expansion of chromatin, thus decreasing or increasing radiation-induced DNA fragmentation. These results raise interesting questions for further studies on the mechanistic links between the sugar-modulated cell volume changes, chromatin structure and radiosensitivity of tumor and normal cells.