The management of patients with pneumonia, the fifth leading cause of death in the United States, remains frequently problematical. Because a precise etiologic diagnosis is often not possible when the patient is first examined, initial antibiotic therapy is often empiric. Problems in early diagnosis include the frequent lack of access to true respiratory secretions, necessary delays in identification of organisms in culture, and contamination of expectorated sputum with oropharyngeal bacteria. Early management must therefore depend on epidemiologic and clinical clues as well as examination of available respiratory secretions. It is generally helpful in the selection of initial antibiotic therapy to consider the disease in two broad epidemiologic categories, community- and nosocomially acquired. As new laboratory and clinical information become available, the appropriateness of the antibiotic therapy must be continually reevaluated.