The authors examined the records of all patients referred for right heart catheterization between 1963-84 because of persistent dyspnoea after one or more episodes of pulmonary emboli. Patients with a history of congestive heart failure, angina, restrictive or obstructive pulmonary disease that could explain their symptoms were excluded. Catheterization was performed 15.8 +/- 24 months after the first suspected episode of pulmonary embolism. Seven of the 29 patients included had resting pulmonary hypertension (PH). All of these had an alveolo-arterial oxygen difference (AaDO2) greater than 25 mmHg. Twenty patients of the group, taken as a whole, had an AaDO2 greater than 25 mmHg. Information was available from 1 month to 5 years later in 6/9 patients with an AaDO2 less than 25 mmHg. In all of them dyspnoea improved or resolved. Information was available in 15/20 patients with AaDO2 greater than 25 mmHg. Three of 8 patients without PH but with an increased AaDO2 on the initial catheterization developed PH within 2 years. Dyspnoea increased in 1 of the remaining five. Four patients who initially had PH developed right heart failure 6 months-3 years later. In the remaining 3, dyspnoea was stable in 1, increased in 1 and one patient died with autopsy evidence of multiple pulmonary emboli. Abnormal oxygenation predicts the presence or subsequent development of PH in patients who are chronically dyspnoeic after pulmonary embolism.