A patient with pronounced dyspnoea and cyanosis was found to have severe hypoxaemia with normal spirographic values. His past history included arterial hypertension, myocardial infarction and phlebitis of the lower limb. Airways resistance was normal, but maximal expiratory flow rates at low lung volume (Flow-volume curves) were reduced, suggesting "peripheral" airways obstruction. This was confirmed by the presence of pulmonary hyperinflation and mechanical non-homogeneity accompanied by unevenly distributed ventilation, as shown by alveolar nitrogen gradient. There was marked hyperventilation with hypocapnia. Since transfer values (measured by the CO single-breath method) and lung distensibility values were normal, emphysema could be ruled out as a cause of obstruction. Analysis of pressure-flow relationship confirmed that the obstruction of peripheral airways was "intrinsic" in character. It could be due to an increase in lung extravascular fluid (interstitial oedema due to left cardiac failure), or to repeated micro-emboli in the lungs, or to hypocapnia, these three mechanisms possibly being associated.