Calculation of aortic valve area by echocardiography is sometimes technically difficult. We tested a modified continuity equation to help measure valve area in those difficult cases. The studies of 105 patients with aortic stenosis were analyzed retrospectively. We calculated aortic valve area by standard continuity equation and by the modified method where Doppler-derived stroke volume was replaced by the difference between diastolic and systolic volume according to Simpson's biplane method of disks. The correlation between the 2 methods was excellent. For patients with left ventricular outflow tract acceleration, modified continuity equation correlated better than standard continuity equation with invasively measured aortic valve area by Gorlin equation. We conclude that the modified method is accurate and becomes an attractive alternative to the conventional continuity equation especially for patients in whom stroke volume calculation by Doppler may be unreliable for technical reasons.