The timing of valve replacement in patients with rheumatic aortic regurgitation is assessed by balancing the mortality and complications associated with the operation and the prosthetic valves against the natural history of the lesion. The time course without surgery is determined by the severity of the volume overload and the gradual deterioration of myocardial function. The authors wished to obtain information both on the haemodynamic recovery achieved after aortic valve replacement in young patients and also on the risks of operation in this group. Twenty patients, in whom the aortic valve was replaced at a mean age of 15 years, were reviewed. An improvement in symptoms and in the cardiothoracic ratio on the chest radiograph occurred in every case, and the voltage measurements suggestive of left ventricular hypertrophy on electrocardiogram diminished in all but two. The left ventricular end-diastolic pressure decreased in the 11 patients who were recatheterised after operaton. The ejection fraction improved in three patients and stayed the same in three others. While there were no operative deaths in this series the incidence of serious morbidity, in terms of myocardial damage at or after operation, was disappointingly high. Early valve replacement to preserve myocardial function is especially attractive in young patients but cannot be advised if the insertion of the prosthetic valve is associated with appreciable myocardial damage.