A low weight has been implicated as one of the major reasons for deferring cardiac surgical procedures on cardiopulmonary bypass (CPB), resorting instead to palliative procedures. The purpose of this study is to analyze the risk factors associated with an increased mortality for surgery using CPB in small infants, and to compare this group with infants weighing over 2.5 kg. Between 1979 and 1990, 60 infants weighing less than 2.5 kg underwent surgery on CPB at our institute. The mean age was 17.9 days (range 1 to 68 days, SD = 16.4) and the mean weight was 2.1 kg (range 1.2 to 2.5, SD = 0.35). Eight had univentricular and 52 had biventricular repairs. The hospital mortality was 16.5% (70% CL 11.5% to 22.7%). The actuarial survival over 100 months was 67.5% (including hospital mortality). Over the same period, 946 patients under the age of 6 months but weighing more than 2.5 kg underwent surgery on CPB with a lower hospital mortality of 7.1% (70% CL 6.3% to 8.0%, p < 0.006). The risk factors associated with an increased risk of early death in the group weighing less than 2.5 kg were analyzed using logistic regression. They were: (1) presence of preoperative metabolic acidosis; (2) univentricular versus biventricular repair; and (3) duration of CPB. Within the less than 2.5 kg group, low weight per se did not affect the outcome adversely. The main cause of late death was the presence of associated medical conditions, notably bronchopulmonary dysplasia and tracheo bronchomalacia.