Ultra-endurance races lead to an enormous energy deficit, and a decrease in body mass in the form of fat mass as well as skeletal muscle mass can be found. The decrease in skeletal muscle mass has been demonstrated in ultra-runners. We investigated therefore, in an ultra-cycling race, whether ultra-cyclists also suffered a decrease in body mass and whether we could find changes in skeletal muscle mass and/or fat mass. The anthropometric method was used to determine body mass, skeletal muscle mass and fat mass in 28 male Caucasian, non-professional, ultra-cyclists before and after a 600 km ultra-cycling race. In order to quantify hydration status, we measured total body water, haematocrit, plasma sodium and urinary specific gravity. In addition, plasma urea was determined as a marker of protein catabolism. Body mass as well as fat mass decreased highly significantly (p<0.01) whereas skeletal muscle mass did not change (p>0.05). The post race minus pre race difference (Delta) in body mass was associated with Delta fat mass (p<0.05). Urea increased highly significantly (p<0.01); however Delta urea was not associated with Delta skeletal muscle mass. We concluded that ultra-cycling in contrast to ultra-running leads to no reduction in skeletal muscle mass.