We measured physical working capacity at a heart rate of 170 bpm (PWC₁₇₀) and diffusing capacity for carbon monoxide (DL_co) 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 DL_co was measured at rest (Dl_{co rest}) and while pedalling at 170 bpm (DL_{co ex}). The PWC₁₇₀, DL_{co rest} and DL_{co ex} were significantly higher in swimmers than in non-swimmers. Repeated measurements in the same subjects show that individual increases in DL_{co rest} and DL_{co ex} were the result of both growth and training.