In patients experiencing an acute episode of hypersensitivity pneumonitis (HP), the alveoli and interstitium show a marked inflammation. The effects of this infiltration with effector cells on gas exchange and the cardiopulmonary system are not well characterized, and there are no data regarding cardiopulmonary exercise testing during hypersensitivity pneumonitis. The aim of this study was to gain new insights into the pathophysiology of acute farmer's lung using cardiopulmonary exercise testing. Cardiopulmonary exercise testing was performed in patients who had farmer's lung (n=21) before and 4 h after a standardized exposure with mouldy hay. Farmers who were asymptomatic for this condition (n=15) served as controls. At baseline, patients who had farmer's lung had a decreased breathing reserve and a greater alveolar to arterial oxygen difference. Following exposure, all of these patients developed hypersensitivity pneumonitis. Compared to the asymptomatic farmers, they showed an increase of alveolar to arterial oxygen difference and functional dead space ventilation during exercise and a reduction of the breathing reserve. In addition, 40% of the asymptomatic farmers demonstrated a pathological increase of the alveolar to arterial oxygen difference during exercise following exposure. In conclusion, our data signify that acute hypersensitivity pneumonitis induces significant changes in pulmonary gas exchange during exercise. Cardiopulmonary exercise testing may help to identify individuals with possible subclinical farmer's lung disease.