Introduction: Measurement of peak consumption of oxygen is widely used to determine need for cardiac transplantation in adults with severe heart failure, with a level below 14.0 ml/kg/min indicating that a patient should be listed. No data exists, however, on its use in children. Hypothesis: We sought to determine whether measurement of peak consumption of oxygen is reproducible, and useful for rationalising decisions over the listing for transplantation of children with end-stage cardiac failure. Methods: We attempted to measure peak consumption of oxygen in all children referred for assessment for transplantation at Great Ormond Street Hospital between July, 2002 and October, 2003. Peak consumption of oxygen was derived from respiratory gas analysis with a medical mass spectrometer. Minimal height for successful use of the equipment was 120 cm. We performed 24 tests in 19 children at a mean age of 12.5, with a standard error of 0.6 years, weight of 40.4 kg, with standard error of 3.3, and height of 147.4 cm, with standard error of 3.0. Of the children, 10 had dilated cardiomyopathy, 1 had restrictive cardiomyopathy, 3 had anthracycline cardiotoxicity, 4 had undergone palliative surgery for complex congenital cardiac disease, and 1 had arrhythmogenic right-ventricular dysplasia. Results: Tests were successful in 16 children, of whom 3 were tested twice, and one was tested three times. Testing was unsuccessful in 3 children, 2 due to lack of co-operation, and 1 due to equipment failure. Of the 21 successful tests, mean peak consumption of oxygen was 21.0 ml/kg/min, with standard error of 1.5. Results correlated well with class in the grading system of the New York Heart Association, with a correlation coefficient of 0.86, though not with echocardiographic assessment of left-ventricular function by fractional shortening measurement. Peak consumption of oxygen was <14.0 ml/kg/min in 4 children, and all have been successfully transplanted. One child with a peak consumption of 22.4 ml/kg/min deteriorated acutely 2 months after testing, and was then too unwell to be retested before a heart became available. The other children, with peak consumption of oxygen >14.0 ml/kg/min, were listed for a medical follow-up, and will be retested as indicated by their clinical condition. All 19 have survived to date. Conclusion: Peak consumption of oxygen can be measured in the majority of children with end-stage heart failure above a height of 120 cm and, as in adults, can be used to provide objective evidence to help in decisions over listing for transplantation.