During the 13 year period from January 1967 to July 1980, the hospital mortality rate for open intracardiac operations in infants in the first 3 months of life was 43 percent (75 deaths among 194 patients), higher than the 22 percent mortality rate (35 deaths in 161 patients) for closed operations in the same age group. The mortality rate was lower late in the experience (p = 0.0001). Poor preoperative condition of the patient increased the mortality rate 87 percent in patients preoperatively acidotic or in shock [preoperative class V]and 22 percent in patients with moderate or severe symptoms but without recent hemodynamic deterioration (preoperative class II or III). The presence of major associated cardiac lesions increased hospital mortality (p < 0.0001). The hospital mortality rate was highest (59 per cent) in infants less than age 1 month, possibly in part because of their sensitivity to the damaging effects of cardiopulmonary bypass. This hypothesis is supported by the association of a long period of cardiopulmonary bypass with increased hospital mortality (p = 0.05) and of total circulatory arrest during profound hypothermia with decreased mortality (p = 0.05). Most deaths (72 percent) occurred in association with acute postoperative cardiac failure. The length of cardiac ischemia (aortic cross-clamping time) was directly related to the probability of cardiac death, unless cold cardioplegia was used. Thirteen percent of the hospital deaths were associated with acute postoperative respiratory failure. Current mortality rates in typical cases without acute hemodynamic deterioration is estimated from these data to be 7 percent (70 percent confidence limits 4 to 12 percent), as a result of the scientific advances made over this period of time. Research into mechanisms of the damaging effects of cardiopulmonary bypass should further improve results in these very young patients. © 1981.