The peak oxygen uptake (VO2(peak)) of 196 healthy children and adolescents aged 8-16 years, and 187 children and adolescents (in the same age range) with congenital heart disease (CHD), was measured using a graded treadmill test (Oslo-protocol). The healthy population was tested to assess the reference values that were to be used in the interpretation of the results obtained from patients with CHD. The results revealed that patients with CHD exhibited lower VO2(peak) values, with declining values for boys after the age of 12-13 years. When separated into different diagnostic groups, on average, patients with a chronic pressure overload of the left ventricle and patients with tetralogy of Fallot have lower VO2(peak) values, but make approximately the same progress with age as healthy subjects. Patients with transposition of the great arteries, however, displayed a marked decline in VO2(peak) after the age of 12-13 years. Whether exercise testing should be included in routine follow-up in patients with CHD, especially those between the ages of 10 and 16 years, when the condition of some patients deteriorates, requires special attention.