The purposes of this study were all to determine if DeltaVO2/DeltaWR is dependent on age, body mass, height and fitness and if DeltaVO2/DeltaWR could discriminate between healthy children and children with a chronic disease that limits O2 delivery or utilization. Four groups were included: muscle disease (Juvenile Dermatomyositis; JDM; n=12), lung disease (Cystic Fibrosis; CF; n=13), Congenital Heart Disease (CHD; (n=13), and healthy children (n=44). All children performed a cardiopulmonary exercise test on a cycle ergometer with respiratory gas analysis. The DeltaVO2/DeltaWR was determined by linear regression using data from unloaded cycling to peak exercise. No associations were found between the DeltaVO2/DeltaWR and age, body mass and height in healthy children. DeltaVO2/DeltaWR was significantly correlated with VO2peak/kg (r=0.44; p<0.01). Children with JDM had lower DeltaVO2/DeltaWR values than healthy children (p=0.02), and DeltaVO2/DeltaWR tended to be lower in CHD and higher in CF (p=0.09 and p=0.08, respectively). DeltaVO2/DeltaWR may be more sensitive for conditions that are characterized by local hypo perfusion (as in JDM), than for conditions that are characterized by impaired oxygen delivery (i. e. CF or CHD).