We measured physical working capacity at a heart rate of 170 bpm (PWC170) and diffusing capacity for carbon monoxide (DLCO) in two groups of children ranging in age from 9 to 17 years. One group was formed of highly trained competitive swimmers and the other of age, sex and size matched untrained counterparts. Bicycle ergometry was used to establish PWC170. The DLCO was measured at rest (DLCO rest) and while pedalling at 170 bpm (DLCO ex). The PWC 170, DLCO rest and DLCO ex were significantly higher in swimmers than in non-swimmers. Repeated measurements in the same subjects show that individual increases in DLCO rest and DLCO ex were result of both growth and training.