In order to examine the prognostic value of pulmonary arterial hypertension (PAH) in patients with moderate and severe acute respiratory failure (ARF). 225 patients with ARF who had been treated with mechanical ventilation and admitted to our IC during a 3-yr period (January, 1983 to January. 1986) were prospectively studied. All 70 (31%) patients with moderate and severe ARF also had some form of hemodynamic or pulmonary instability, and were monitored with a pulmonary artery catheter. Of these 70 patients. 38 (54%) had PAH (mean BP 29 ± 6 mm Hg); their mortality was 79% (30/38). The rest of the patients (n = 32) did not have PAH (mean BP was 15 ± 3 mm Hg) and their mortality was 44% (14/32) (p < .01). Thirty patients met all the criteria for adult respiratory distress syndrome (ARDS). and their mortality was 707% (21/30); all of them were included among the 38 PAH patients. ARDS patients who died had a significantly higher pulmonary vascular resistance and a significantly lower cardiac index than patients who survived (p < .001). We conclude that PAH (present in all our ARDS patients) is a good predictor of mortality in ARF of diverse causes.