One hundred thirty-nine infants and small children less than 48 months old were studied during the first 72 hours after intracardiac surgery for a variety of lesions. The hospital mortality rate was 19.4% (27 patients); 16 of those dying succumbed from acute cardiac failure. Deaths from acute cardiac failure were commonest in patients with low cardiac index (CI), and a continuous probability curve relates the two. The mean CI for all patients was 2.51 plus or minus 0.794 l-min minus 1-m minus 2, and that for individual patients varied between 0.6 and 4.9. Cardiac output normalized by surface area (cardiac index) correlated weakly with age. cardiac output normalized by weight did not correlate with age; Mean mixed venous oxygen partial pressure (P-vO2)varied between 17 and 60 mm Hg, and the weighted mean for the group of 80 patients in whom it was obtained was 33.1 plus or minus 6.57 mm Hg. CI and P-vO2 were only slightly related. Acute cardiac deaths occurred more frequently in patients with low P-vO2. Acute cardiac death was more reliably predicted using CI and P-vO2 together than either alone. The average of the mean arterial pressure was 80.5 plus or minus 2.53 mm Hg, and this did not correlate with CI. Mean average systemic vascular resistance was 30.0 plus or minus 8.395 SRU. We conclude that in such patients treatment should be directed toward keeping CI greater than 2.0 l-min minus 1-m minus 2 and P-vO2 greater than 30 mm Hg; that the possibility of maintaining adequate CI and P-vO2 is not related to the age of the patient but is related to the malformation treated; and that a therapeutic trial of reducing left ventricular afterload is indicated in some of these patients.