In order to obtain information about the extent and duration of the changes in the leukocyte and differential counts following cardiac surgery done with and without extracorporeal circulation in the absence of any infection, 28 children were studied serially during the first ten postoperative days. In patients operated without extracorporeal circulation, normal values are reached after 24--48 h; in patients operated with extracorporeal circulation, normal values are reached only after 4--8 days, depending on the length of extracorporeal circulation.