A review of the medical records of 123 persons with Legionnaires' disease hospitalized in the 1976 Philadelphia epidemic showed that the manifestations of infection ranged from mild grippe to a severe pneumonia that also involved other organ systems. Early in the illness, constitutional symptoms predominated. Fever, malaise, myalgia, rigors, confusion, headache, and diarrhea were usually followed by nonproductive cough and dyspnea. Physical examination showed few abnormalities other than rales. Moderate leukocytosis with left shift, elevated erythrocyte sedimentation rate, elevation of serum levels of liver enzymes, and hematuria and proteinuria were characteristic. Chest radiograph showed patchy, often nodular, areas of consolidation. Progression of pneumonia led to respiratory failure and the need for mechanical ventilatory assistance for 19 patients; renal failure, primarily after shock, occurred in 18 persons. Twenty-six patients died. Treatment with erythromycin or tetracycline resulted in the lowest case-fatality ratios, but the associations were not statistically significant.