Antibiotic prescribing information was prospectively collected on 1822 hospitalized patients treated for suspected or documented bacterial pneumonia. Antibacterial therapy with a single antibiotic was employed in more than 50 percent of the patients, with cefazolin, cefuroxime, ampicillin, and ceftriaxone sodium representing the most commonly employed agents. Combination therapy using two antibiotics was employed in approximately 30 percent of patients with the aminoglycosides, particularly gentamicin, used extensively. A satisfactory outcome was achieved in approximately 80 percent of patients with a community- or institutional-acquired pneumonia; only 66 percent of nosocomial pneumonias had a satisfactory outcome. An important observation was the apparently common practice of switching patients to an oral antibiotic regimen after an average of seven days of antibiotics and subsequently discharging the patient. No difference was observed in the patterns of clinical response or duration of therapy for culture-positive versus culture-negative patients. The results of this surveillance program can serve as a basis for comparison of institution-specific drug utilization evaluation programs.