Summa yIt is well known that complement activation plays a key role in the development of pulmonary insufficiency; it is also well known that cortisol suppresses complement activation. By measuring the plasma concentrations of C3a and C5a, we investigated peri-operative changes in the activation of the complement system in 18 infants undergoing elective abdominal surgery. We also measured plasma cortisol concentrations to assess the peri-operative relationship between complement activation and the stress produced by multiple perioperative factors following two anaesthetic techniques. Eighteen infants ranging in age from 1 to 11 months were randomly divided into two groups according to the anaesthetic technique used: Group 1 consisted of nine infants in whom general anaesthesia was maintained with halothane and nitrous oxide (N20) in oxygen, while Group 2 consisted of nine infants in whom general anaesthesia was maintained with fentanyl and N 2 0 in oxygen. Plasma C3a and C5a concentrations were higher in the fentanyl group than in the halothane group during the peri-operative period. Plasma cortisol concentration, in contrast, was lower in the fentanyl group both during and after surgery. The post-operative clinical course showed no significant intergroup differences between the two groups throughout the study. These observations suggest that the difference in peri-operative complement activation between the halothane and the fentanyl groups may have been due, in part, to different peri-operative stress responses. However, further studies are required to elucidate the precise causative mechanisms and the clinical implications of complement activation in infants.