Myocardial blood flow in the region supplied by the right coronary artery was estimated in 36 patients after selective Xenon-133 injection into the artery. Its values were analyzed in relation to coronary driving pressure, diastolic coronary vascular resistance, and right ventricular triple product (tension-time index, TTI). Myocardial blood flow in the region supplied by the right coronary artery was increased in patients with moderately elevated pulmonary artery pressure (4.13 to 8.0 kPa; 31 to 60 mmHg). This increase was due to a significant lowering of diastolic coronary vascular resistance. In patients with severe pulmonary hypertension (above 8.00 kPa; >60 mmHg) the greatly increased value of TTI was not accompanied by a further augmentation of myocardial blood flow. This incommensurability between metabolic demand of the myocardium, reflected in TTI value, and the abililty to increase blood flow in the region supplied by the right coronary artery could be one of the causes of myocardial ischaemia and anginal pain in patients with extreme pulmonary hypertension.