Abstract: Objective To investigate the factors for the secular changes in incidence of JEV in Shanghai.Methods The ecological factors of JE,such as meteorological records,area of rice field,size of pig populations and mosquito population density during the recent 50 years were studied and compared to the secular changes in JE incidence rate during the same period.Results It was revealed that the increase in area of rice field and pig raising which might be a cause for the JE epidemic in Shanghai,1965-1967,could not be associated with the dramatic decline in JE incidence rate after 1968,which could be explained solely by the universal childhood vaccination against JEV.Conclusion JE vaccination was highly effective for control of JE and should be kept on in the foreseeable future.