We evaluated the efficacy of 3 treatments for chronic obstructive pulmonary disease in horses: prednisone (400 mg/horse, PO, daily; n = 7), methyl sulfonmethane (10 g/horse, PO, q 12 h; n = 6), and clenbuterol hydrochloride (0.4 mg/horse, PO, q 12 h; n = 7). A fourth group acted as controls (n = 6) and was not treated. The treatment period lasted 10 days. Each horse was a member of 2 different groups for 10 days, separated by an 18-day interval of no treatment. All horses were housed together in an outdoor pen without bedding. Horses were fed alfalfa/grass hay mix ad libitum from a large feeder. The same batch of hay was fed throughout the study. Multiple physical and laboratory variables were monitored prior to, during, and at the end of each 10-day trial period. Changes in lung sounds, respiratory effort, degree of anal movement, nasal discharge, temperature, respiratory rate, or heart rate were not significant. Changes in arterial blood gas tensions, tracheal wash or bronchoalveolar lavage cytologic findings, or phagocyte function were not significant. All horses were tachypneic and most were tachycardic. The median value for PaO2 was below normal for all horses. All tracheal wash and most bronchoalveolar lavage cytologic findings represented a suppurative response. Negative linear correlation was observed between PaO2 and degree of respiratory effort in these horses (eg, as PaO2 decreased, the degree of respiratory effort increased).