The pulmonary-to-systemic blood flow ratio (Qp/Qs) is critical in the appraisal of the need for surgery in patients with left-to-right shunts. Because of the drawbacks present with conventional Fick, indicator dilution, and radionuclide techniques of Qp/Qs measurement, we sought to determine whether an extension of thermodilution technology could accurately predict Qp/Qs. We studied 30 children with clinically suspected or postoperative atrial or ventricular septal defects. The thermodilution temperature curves from the pulmonary artery were printed on a strip chart recorder after right atrial injection of iced solution. Gamma variate curve fitting and area analysis were used to determine Qp/Qs. The correlation between Fick and thermodilution Qp/Qs values was excellent (r = 0.95). The thermodilution technique was rapid, and did not require either arterial entry, radiation after venous catheter placement, or multiple sampling.