A case report and review of 13 cases ofPasteurelia multocidaempyema in the English literature demonstrate the apparent increased pathogenicity of this organism in patients with chronic lung disease. The infections were marked by their indolence, minimal febrility, blood-tinged pleural fluid, and predilection for the elderly. The was no characteristic roentgenographic pattern. Animal exposure commonly preceded the illnesses. It is postulated that the disease occurs in patients with impaired pulmonary defenses who acquire the organism via the inhalation of infectious droplets aerosolized from the oropharynx of animals whereP multocidais part of the normal flora. Although most isolates were penicillin sensitive, there was significant mortality.