Few reports on human cardiac functional development exist, although this information is important for managing paediatric heart disease. The work and the energy usage of the heart was measured in children. A total of 58 patients (aged 1-19 years) with a history of Kawasaki disease without coronary sequelae underwent cardiac catheterization to obtain haemodynamic data and to measure myocardial oxygen consumption. Myocardial oxygen consumption (ml/min) (y = 0.63 x + 3.6, r = 0.86, P < 0.0001, x = age) and left ventricular minute work (kg m/min) (y = 0.46 x 2.4, r = 0.84, P < 0.0001, x = age) correlated positively with age. However, left ventricular minute work per body surface area (age: 2-5 years, 5.8 +/- 0.34 kg m/min/m2; age: 6-10 years, 6.9 +/- 0.59 kg m/min/m2; age: 11-15 years, 5.9 +/- 0.51 kg m/min/m2; age 16-19 years, 6.5 +/- 0.29 kg m/min/m2; and myocardial efficiency (age: 2-5 years, 40.1 +/- 4.4%; age: 6-10 years, 42.4 +/-3.9%; age: 11-15 years, 45.9 +/- 4.1%; age: 15-19 years, 42.3 +/- 6.6%) remained constant throughout childhood.In spite of the structural immaturity of the developing heart, the myocardial oxygen consumption per body surface area and myocardial efficiency led by the cardiac work are the same in adults and in children older than 1 year of age.