The authors studied the pulmonary haemodynamic response to exercise in eleven patients with toxic oil syndrome (TOS) (mean age 38.3 +/- 15.7 years; 10 women, 1 man) and abnormal pulmonary diffusing capacity (39.1 +/- 10.3% of predicted value) without clinical evidence of pulmonary hypertension. Eight patients had normal pulmonary pressure at rest (mean PAP less than 25 mmHg) and three showed mild pulmonary hypertension. After exercise the mean PAP rose to 35.3 +/- 11.5 mmHg from a basal value of 20.72 +/- 3.8 mmHg (p less than 0.01). Only four patients did not develop pulmonary hypertension during exercise. Pulmonary artery oxygen saturation decreased from 72.9 +/- 1.9% at rest to 52.3 +/- 10.1% during exercise (p less than 0.01). In conclusion, in this subset of TOS patients, an early diagnosis of their subclinical pulmonary hypertension can be made on the basis of the presence of dyspnoea and abnormal pulmonary diffusing capacity for carbon monoxide and can be then confirmed with the exercise haemodynamic test.