Pulse oximetry is a noninvasive technique for measuring O₂ saturation (SpO₂) continuously. We applied pulse oximetry to 9 pediatric patients with tetralogy of Fallot during shunt surgery. Arterial oxygen tensions (PaO₂) and saturations (SaO₂) were also measured at the time of postinduction, just before the shunt, after the shunt and at the end of the operation. The SpO₂ and SaO₂ levels were identically changed in all 4 periods. The PaO₂ was increased a little without statistical significance after the shunt procedure and at the end of the operation compared with the values before the shunt. However, SaO₂ values increased with statistical significance after the shunt procedure and SpO₂ values also showed similar increases with significance. In conclusion, continuous monitoring of SpO₂ by pulse oximetry, instead of PaO₂, is a very useful and reliable method to assess the improvement of perfusion after shunt, particularly in cyanotic cases.