Pleuropneumonia is a life threatening disease in the horse that requires prompt and aggressive treatment. This paper will present a case of pleuropneumonia in a Thoroughbred brood mare, which developed severe complications of endotoxemia, laminitis, disseminated intravascular coagulation and thrombophlebitis. Treatment, outcome and pathophysiology are discussed. The etiology of pleuropneumonia involves an inciting stress, contamination and infection of the lung paranchyma with oropharyngeal microorganisms and extension of the infection to the pleural membranes. Stress factors include shipping, strenuous exercise, anesthesia, primary viral infection and immunosuppressive disorders. Significant pleural effusion and diminished respiratory capacity results. Anaerobic infection of the lung and pleura occur approximately 5-7 days after the initial signs of pneumonia. Secondary complications are common. A delay in aggressive treatment and the presence of an anaerobic infection lend to a poor prognosis.