Ozonotherapy is a complementary medical approach in the treatment of resistant infections, immune deficiency syndromes, orthopedic pathologies and vascular diseases. The criticism of this method is associated with potentially harmful effects of ozone on cells. The aim of this study was to investigate the influence of ozonated autohemotherapy (O3-AHT) on the cellular response of the immunologic system represented by cytotoxic activity of natural killer cells. 12 hemodialyzed patients (8 M, 4 F) aged 64.8 +/- 7.6 years with peripheral arterial disease as the main reason for the treatment with O3-AHT were examined in a prospective, placebo controlled, single blind study. They received 9 sessions of autohemotherapy without ozone exposure as a placebo-control and subsequent 9 sessions of O3-AHT. The procedures were performed 3 times a week, just before hemodialysis session. Ozone-oxygen gas mixture with ozone concentration of 50 microg/ml produced by ozone generator (ATO3, KrioMetrum, Poland) was used during O3-AHT Natural killer cell activity was measured using lactate dehydrogenase release assay There was no statistical difference between natural killer cell activity (%) at the baseline (16.78 +/- 8.07), after nine sessions of control autohemotherapy (15.98 +/- 6.67), and after nine sessions of O3-AHT (18.26 +/- 8.82). In conclusion, our findings showed that O3-AHT in a dose of 50 mg/mL does not have any significant influence on natural killer cell function in hemodialyzed patients.