A noninvasive method for measuring systemic and pulmonary blood flow using Doppler velocimetry combined with 2-dimensional (2-D) echocardiography has been developed. High correlations were found between Fick- and Doppler-derived indexed measurements of systemic and pulmonary flow as well as the pulmonary to systemic flow ratio in 33 patients undergoing cardiac catheterization (systemic flow [n = 28], r = 0.78; pulmonary flow [n = 21], r = 0.88; Qp/Qs ratio [n = 24], r = 0.85). The random errors of the 2 methods were not significantly different. Outflow tract obstruction, semilunar valve regurgitation, and patent ductus arteriosus were the only lesions in which limitations to the use of this method were encountered. We anticipate that this method will be of use in initial and serial evaluations of adult and pediatric patients with low cardiac output or intracardiac shunts.