Eight of altogether 9 Norwegian boys with cystic fibrosis (CF), aged 15-17 years, and 8 healthy boys of the same age underwent pulmonary function and bicycle exercise testing. Although the CF boys showed large individual variations, the two groups showed great differences in mean body height and weight, pulmonary function variables and maximum oxygen uptake. The maximum oxygen uptake for the CF boys ranged from 40 to 125% (mean 79%) of predicted values. Increased ventilatory equivalent for oxygen was a characteristic finding during rest and exercise for the CF boys, and their oxygen uptake was higher during rest and corresponding work loads compared with the controls. As physical training is beneficial in cystic fibrosis, such exercise tests are of value for prescribing individual training programs and in evaluating their effects.