Noninvasive determination of right ventricular systolic pressure was attempted in 27 patients with ventricular septal defect based on the peak velocity of left-to-right shunt flow as measured in the right ventricle by a continuous wave Doppler technique. The systolic pressure gradient between the ventricles (delta p: mmHg) was calculated according to the simplified Bernoulli's formula, delta p = 4 V2, where V (m/sec) is the peak velocity of the left-to-right shunt flow. Right ventricular systolic pressure was determined by subtracting delta p from the systolic blood pressure measured in the upper arm, which was regarded as the left ventricular systolic pressure. The peak velocity of left-to-right shunt flow could be measured in all patients except one with muscular ventricular septal defect, and values ranged from 0.6 to 4.8 m/sec. The peak velocity of left-to-right shunt flow decreased inversely with the increase in right ventricular systolic pressure. The values of right ventricular systolic pressure determined by the continuous wave Doppler technique correlated highly (r = 0.820) with those determined by cardiac catheterization. The peak velocity of left-to-right shunt flow also showed high negative correlations with the pulmonary to systemic pressure ratio (r = -0.876) and pulmonary to systemic resistance ratio (r = -0.855). These results indicate that the continuous wave Doppler technique is clinically useful for determination of right ventricular systolic pressure as well as the pulmonary to systemic pressure ratio and pulmonary to systemic resistance ratio.