We performed bronchoalveolar lavage (BAL) in 2 groups of ponies. Principal ponies had a history of heaves, a disease characterized by recurrent airway obstruction and airway hyperreactivity when ponies are housed in a barn and fed hay; control ponies had no history of airway obstruction. Ponies were paired (principal and control), and BAL was performed after 2 months of being pastured when principal ponies were in clinical remission (Period A), after barn housing when principal ponies had acute airway obstruction (Period B), and after a 1- and a 2-wk recovery phase of pasture grazing (Periods C and D). At Periods A, C, and D, white blood cell counts and immunoglobulin/albumin ratios in peripheral blood and BAL fluid of principal and control animals did not differ. Peripheral blood white cell counts and immunoglobulin/albumin ratios were unaffected by barn exposure and return to pasture. However, at Period B, neutrophil numbers in the BAL fluid were increased in the principal but not in the control animals. The IgG/albumin ratio of the principal animals increased at Period C. We conclude that in ponies with a history of heaves, barn exposure results in increased neutrophils in BAL fluid. In this pony model of lung disease, pulmonary leukostasis follows a time course similar to that of airway obstruction and airway hyperreactivity.