One hundred four autopsy cases with previously diagnosed pneumonitis were examined for evidence of Legionnaires' disease. The peak epidemic months of July, August, and September in the five years before the 1977 Vermont epidemic were chosen for study. The bacterium, Legionella pneumophila (serogroup 1) was demonstrated in lung tissue by direct immunofluorescence and the Dieterle silver impregnation stain. There was no clustering of Legionnaires' disease in any one year., The clinical presentation and pulmonary pathology were similar to that of Legionnaires' disease previously reported in Vermont. Seven out of 104 cases were identified as previously undiagnosed Legionnaire's disease. In this time frame, it can be concluded that the disease has been endemic in Vermont.