The genotoxicity study of ornidazole (ONZ) was carried out on human lymphocyte chromosomes, using sister chromatid exchange (SCE) and micronucleus (MN). Thirty-two patients with Entemoeba histolitica infection who received 1000 mg/day for 10 days were included in this study. SCE and MN were measured before and after therapy. A statistically significant increase was observed in the SCE (P < 0.001) and MN frequencies (P < 0.001) after ornidazole therapy. It was concluded that ONZ has a potential geno- and cytotoxic effect in human peripheral lymphocyte cultures. For this reason, further, detailed studies are needed to elucidate the ONZ mechanism of genotoxicity and its carcinogenic potential.