During 1975, there was widespread occurrence of St. Louis encephalitis (SLE) in Illinois where 578 cases and 47 deaths were reported. As a result, a statewide surveillance program was established to provide with early warning of high or increased levels of SLE virus transmission. Evidence of transmission was first detected in birds collected in mid-June in Central and Southern Illinois and reported to the public in early July. The onset of the first human case was in mid-July, but not confirmed until the third week of August. The organization, activities, results and value of this program are discussed.