Summertime pneumonias in Philadelphia during 1976 were studied epidemiologically, and the epidemiologic, clinical, and laboratory features of pneumonia cases serologically positive for Legionnaires' disease were compared with features of serologically negative cases. Both groups were similar in many respects, but in patients with Legionnaires' disease diarrhea and neurologic findings were significantly more frequent (P = 0.01 and P = 0.05 respectively). A diagnosis of Legionnaires' disease was also suggested by an elevated serum creatinine phosphokinase level (P = 0.02) and the presence of occult blood in the urine with fewer than six erythrocytes per high-power field. Abnormalities in renal function tests or liver function tests were commoner in patients with Legionnaires' disease (P = 0.05). Radiographic features, however, could not be used to separate pneumonia cases. The high frequency of extrapulmonary manifestations involving the gastrointestinal tract, the central nervous system, kidneys, and liver suggests that Legionnaires' disease is a multisystemic disorder possibly caused by a toxin-producing organism.