Complement activation was evaluated prospectively in serial serum and plasma samples from 30 children, who underwent cardiopulmonary bypass operations. Each patient showed a decrease in total hemolytic C levels and increase in C3 activation product (C3a desArg and C3bi/c) levels. In 11 of the 30 patients surgery was associated with a complication: death (2), respiratory failure (RF, 9), septicaemia (4), postpericardiotomy syndrome (PPS, 1) and pancreatitis (1). Patients suffering from RF were younger, underwent longer perfusions and had somewhat more extensive changes in the C system, although no direct correlation between high C3a desArg levels and RF existed. Instead, the increase in C3a desArg was in linear correlation with the perfusion time. Most complications during the latter postoperative period were associated with distinct episodes of lower grade C activation.