BACKGROUND: DNA methylation is an epigenetic phenomenon known to play an important role in the development of cancers, including colorectal cancer (CRC). Aberrant methylation of promoter regions of genes is potentially reversible, and if methylation is important for cancer survival, demethylation should do the opposite. To test this we have addressed the hypothesis that DNA methyltransferase inhibitors (DNMTi), decytabine and zebularine, potentiate inhibitory effects of classical anti-CRC cytostatics, oxaliplatin and 5-fluorouracil (5-FU), on survival of CRC cells in vitro. RESULTS: Isobole and median effect analysis revealed that decytabine shows potent synergistic interaction with oxaliplatin and 5-FU and that this is probably not the class effect of DNMTi as zebularine shows strong antagonistic interaction with oxaliplatin. The synergistic combination treatment was also applied to the cultures to investigate their mechanisms of action. We have shown that combinations of decytabine with cytostatics produced dose-dependent growth inhibition and treatment-induced apoptosis. CONCLUSION: The observed synergism between decytabine and cytostatics is most probably related to the augmented apoptotic signal and allowed for significant (both biologically and statistically) reduction of the cytotoxic doses of cytostatics used.