The resting cardiac output was measured in 151 healthy persons (109 males and 42 females) of 4 to 78 years of age by the earpiece dye-dilution method by using an automatic calibration device of dye concentration. Cardiac index had a significantly inverse correlation to age in 105 cases with the age higher than 20 years: r=-0.491 and y=4.874-0.023 x, where y is cardiac index (liters/min/m²) and x is age (years). Stroke index also decreased linearly by aging throughout all ages: r=-0.426 and y=71.0-0.26 x, where y is stroke index (ml/m²) and x is age. There was no significant difference between males and females. A significant difference was found between the young group below 14 years of age and the other group older than 15 years when cardiac output was correlated to body sizes such as height, weight and surface area. The former which is corresponding to the growth age showed significantly higher cardiac output than the latter. From these results, age-matched normal predicted values were presented. It is suggested that the measured cardiac index and stroke index should be taken as abnormal if those values deviate from the normal value by 30% or more.