In 19 children with Kawasaki disease without any cardiac sequelae the coronary sinus was cannulated via the femoral vein with a specially designed flow catheter and coronary sinus blood flow was measured by the continuous thermodilution method. There was a statistically significant positive correlation between coronary sinus blood flow and age, body surface area, and left ventricular mass, but coronary sinus blood flow per left ventricular mass (100 g) was negatively correlated with age, body surface area, and left ventricular mass. Coronary vascular resistance was negatively correlated with age, body surface area, and left ventricular mass. Younger children require a much greater coronary blood flow per left ventricular mass and have a higher coronary vascular resistance than older children and adolescents. These results may indicate that coronary blood flow is less efficient in childhood than in adolescence or adulthood.