A triathlete has to complete a hard endurance effort in aerobic circumstances. This requires important cardiovascular, haemodynamic and metabolic adaptations which alter the percentage body fat. This study included 52 triathletes and 22 control persons. The anthropometric data of the two groups were similar. All the subjects underwent the same extensive non-invasive cardiac exploration with two-dimensional cardiac echo-doppler examination. Maximal incremental exercise tests with determination of lactate and the ventilatory threshold were done on bicycle and on treadmill. Three different methods determined the percentage of body fat: 4 and 12 skin fold method, bioelectrical impedance analysis and dual energy X-ray absorptionmetry. The results showed important structural and functional heart changes in the triathletes. These changes caused distinct heamodynamic adaptations so that the maximal performing capacity and the aerobic capacity could be forced up largely. The haemodynamic adaptations were connected with changes in the percentage body fat in triathletes. The determination of the 12 skin fold measurements enabled us to distinguish the triathletes with better competition results from the inferior triathletes. It is concluded that the method of 12 skin fold measurements gives the most reliable results and requires only a limited instrumentarium. Moreover, this examination can be performed correctly and easily in all circumstances.