Haemodynamic studies were performed in 30 patients at a mean interval of 43.4 (range 27 to 59) months after aortic valve replacements with pericardial xenografts. Five valve sizes--19, 21, 23, 25, and 27 mm annulus diameter--were used. Of these 30 patients, 16 had preoperative haemodynamic investigations. Comparison of the pre- and postoperative data showed a marginal but significant increase in cardiac output (P less than 0.05). There was a significant reduction in the pulmonary wedge and left ventricular end-diastolic pressures at rest and on exercise (P less than 0.05 and P less than 0.01). At the postoperative study the mean peak systolic gradient was 8.3 mmHg at rest and 12.3 mmHg on exercise in patients with the smallest xenograft inserted (19 mm) and decreased with each corresponding increase in graft size, so that across the 27 mm graft it was hardly measurable. The calculated xenograft surface area ranged from 1.1 to 2.1 cm2 at rest and 1.3 to 2.4 cm2 during exercise. Aortic root angiography showed competent valves in all 30 patients, but a grade 1 to 2 perivalvular leak was shown in 8 patients. This long-term study has indicated significant circulatory improvement and very small transvalvular gradients in patients with aortic pericardial xenografts.