In a prospective study of community-acquired pneumonias, 30 patients were diagnosed with Legionnaires' disease in 15 months. Clinical, laboratory and radiologic features of these patients are reviewed and compared with those who have pneumococcal pneumonia. Alcoholism, history of smoking, previous antimicrobial therapy, gastrointestinal and neurologic manifestations, elevations of serum transaminases, alkaline phosphatase and creatinine levels were more frequent in pneumonia due to Legionella pneumophila than in pneumococcal pneumonia. The presence of respiratory failure and radiologic progression were common findings that suggested L pneumophila as the etiologic agent of a community-acquired pneumonia. Development of respiratory failure was associated with involvement of several lobes and isolation of L pneumophila in any specimen. In 21 of 30 patients with Legionnaires' disease, L pneumophila was isolated from respiratory specimens. Overall mortality was 10 percent, but it increased to 27 percent in patients not treated with erythromycin initially.