Eight horses (mean weight 438 kg) with chronic obstructive pulmonary disease (COPD) were studied for clinical signs, ventilation/perfusion relationships (V̇A/Q̇) and lung morphology. Four horses were killed and necropsied after the study. In horses with COPD, minute ventilation was almost twice as high as normal, whereas PaO2 was significantly decreased. Cardiac output was normal, but pulmonary artery pressure and pulmonary vascular resistance were significantly increased. The V̇A/Q̇ distribution was abnormal with an increased scatter of V̇A/Q̇ ratios. However, shunt (V̇A/Q̇=0) was increased in one horse only. Ventilation of dead space and high V̇A/Q̇ regions (V̇A/Q̇>10) were increased markedly and comprised 74 per cent of minute ventilation. On lung biopsy, all horses showed chronic bronchiolitis with epithelial hyperplasia and metaplasia. Diffuse acinar hyperinflation also was evident at necropsy. There was a significant inverse correlation between the respiratory rate and the fractional ventilation of normal V̇A/Q̇ regions. Also, there was a significant agreement between the extent of bronchiolar epithelial hyperplasia in necropsy specimens of lungs and the degree of ventilation of high V̇A/Q̇ regions and dead space. The findings suggest that hyperinflation of the lung due to obstructed airways may be the common denominator of increased ventilation of high V̇A/Q̇ regions and dead space ventilation and of elevated pulmonary artery pressure in horses with COPD from chronic bronchiolitis.