The cytogenetic damage induced by municipal landfill leachate was studied using chromosomal aberration (CA) in mouse bone marrow assay. Results show that leachate samples collected in different seasons decreased the mitotic index (MI), and caused significant increases of CA frequencies in treatment concentration (Chemical oxygen demand (COD) measured by the method of potassium dichromate oxidation, COD(Cr))-dependent manners. Compared with the negative control, reductions of the MI of 54 and 38% were detected for the highest leachate concentration (COD(Cr) 320mg/L) in mouse bone marrow treated with both samples. The frequencies of CA increased significantly with increasing concentrations of sample 1 from COD(Cr) 40 to 320mg/L, and from 80 to 320mg/L after exposure to sample 2. In addition, a seasonal difference of MI and CA frequencies induced by leachate was observed. The results confirm that leachate is a genotoxic agent in mammalian cells, and imply that exposure to leachate in aquatic environment may pose a potential genotoxic risk to mammals and humans. The results suggest that the CA in mouse bone marrow bioassay is efficient in genotoxicity studies of leachate on mammals, and that there appears to be a correlation between the genotoxicity in mammal system and the chemical measurement (COD(Cr)) of leachate. The results also indicate that different discharge guidelines and environmental quality standards should be established for leachates discharged from landfills to aquatic environment in different seasons.