In 125 consecutive patients, aged greater than or equal to 10 years (mean 27, range 10-64), evaluated for isolated ventricular septal defects (VSD) the initial prevalence of aortic regurgitation (AR) was 12/125. Forty-one patients were operated on, and post-operative mortality was 3/6 in patients operated on for VSD and AR and 1/35 in those operated on for VSD alone. All but one of the patients have been followed until death or beyond the age of 30 years (mean 42, range 31-73) and a prospective restudy has been performed after a mean observation time of 15 years (range 4-21). The incidence of new cases of AR arising during this period was 10/111. AR was severe in 5 cases (one died from heart failure), moderate in 1 and mild in 4. Surgical repair of AR and VSD was performed in 3 cases. Common characteristics of patients who developed AR were advanced age, male sex, history of bacterial endocarditis, small subaortic VSDs and tricuspid aortic valves without prolapse. Echocardiography revealed larger aortic root diameter (p less than 0.001), increased eccentricity factor (p less than 0.001) and increased left ventricular dimensions (p less than 0.02) in those with complicating AR. AR in adults with VSD may have an unpredictable clinical course; it may be difficult to assess clinically and the need for close clinical control is emphasized. Echocardiography remains of considerable value in selected cases.