Retrospectively, we clinically compared community-acquired cases of Legionnaires' disease, pneumococcal, and mycoplasmal pneumonias. Relative to pneumococcal and mycoplasmal pneumonias, patients with Legionnaires' disease were significantly more likely to present with unexplained encephalopathy, hematuria, and elevation of serum glutamic-oxalacetic transaminase than were those with pneumococcal and mycoplasmal pneumonias. We found upper respiratory symptoms infrequently in patients with Legionnaires' disease, and progression of pulmonary infiltrates occurred commonly. Differentiation of Legionnaires' disease pneumonia without encephalopathy from pneumococcal and mycoplasmal pneumonias may be difficult because of demographic, clinical, laboratory, and radiographic similarities.