A detailed review was made of 180 patients with severe aortic regurgitation of rheumatic origin. Of these patients, 110 underwent aortic valve replacement. Thirty-nine clinical and haemodynamic factors were studied in an attempt to define those associated with (1) death before surgery, (2) a higher incidence of complications and hospital mortality after surgery, and (3) an unsatisfactory longer-term result of surgery. Only heart failure, radiographic heart size, left ventricular hypertrophy, and ventricular premature beats were associated with death before surgery. No factor predisposed to surgical complications and only preoperative factors associated with an unfavourable result after surgery were advanced heart failure, cardiomyopathy, extreme cardiomegaly, and ventricular premature beats. It is concluded that the indications for operation are: a cardiothoracic ratio of greater than 0-60, or a history of heart failure combined with electrocardiographic evidence of extreme left ventricular hypertrophy. Operation may be safely postponed if these indications are not met, though the presence of ventricular extrasystoles or evidence of independent myocardial disease are further factors which should influence the decision.