Abstract: Objective To explore long-term effect of endemic arsenism on immune function among residents and to providie evidence for prevention and treatment of arsenism.Methods With cluster sampling and typical investigation,the subjects were divided into five groups:mild,moderate,and severe patient group,external control and internal control group. The patient groups and the internal control group were selected in the arsenism areas 5 years after drinking water improved. Immunoglobulin G(IgG)and lysozyme(LSZ)were measured.The data were analyzed with SPSS13.0.Results There were 252 people surveyed in five groups.The water arsenic in the arsenism areas was below the national standard for drinking water quality(≤0.05 mg/L).Urine arsenic contents were significantly different among the five groups(H=2.698,PM> 0.05).The concentrations of IgG were significantly different among the five groups(H=52.923,P<0.01).The average content of IgG in external control group was 11.16 g/L and significantly higher than that of in the other groups(P<0.05). The concentrations of lysozyme(LSZ)were significantly different among the five groups(H=34.998,P<0.01).The average of LSZ in external control group was 13.57μg/ml and significantly higher than that of in the other groups(P<0.05).Conclusion Arsenic could induce long-term effect on immune function among the residents living in the area five-year after water improvement.People in arsenism areas after water improvement must be surveyed and water improvement needs to be strengthened.