The prevalence of Keshan disease and the ecological environment in three villages of Hulun Buir League,Inner Mongolia where Keshan disease was severely prevalent,were monitored for three years(1987-1989).A non-affected village was selected for control. Keshan disease incidence came down during those three years.The average incidence decreased from 16.23% in the spring of 1987 to 14.78% in the winter of 1987.But the hair Se appeared rising trend.There is a negative correlation between the decrease of Keshan disease incidence and the increase of hair Se.The ecological environment of affected areas is still in low Se situation.Low Se in food of affected areas leads to low Se flux in human beings,which is the main reason for low hair Se in affected areas.