Background: This study was undertaken in order to investigate any cardiopulmonary changes caused by the surgical procedure during a Nuss operation in patients with pectus excavatum. Methods: Thirty patients undergoing the Nuss operation were divided into two groups. The patients were randomly assigned to either Group-V (n 15) which volume controlled ventilation was applied, or to Group-P (n = 15) where pressure controlled ventilation was applied. Mechanical ventilation provided a tidal volume to be set at approximately 10 ml/kg and a frequency set to keep ETCO. In range, between 32 mmHg and 38 mmHg. During the surgical procedure, we recorded expired volume (VE) and inspired maximal pressure (Pmax) changes according to the ventilator mode, and then also recorded changes in vital signs. Results: Heart rate and blood pressure had no significant changes in either groups. In Group-V, during rotation of the metal bar, there was a significant decrease in expired volume (P < 0.001), changes of inspired maximal pressure gradually increased with the operation (P < 0.001). In Group-P, rotation of the metal bar caused a significant decrease in expired volume (P < 0.001). Conclusions: During the Nuss operation, there should be careful observation of changes in the vital signs, expired volume and maximal pressure during insertion and rotation of the metal bar. (Korean J Anesthesiol 2001; 41: 178-183)