OBJECTIVE: To assess the change in body composition and hydration status in cyclists in a mountain bike ultra-marathon. DESIGN: Prospective observational field study. SETTING: "Swiss Bike Masters" 2008 over 120 km with a total climb of 5000 m in altitude. PARTICIPANTS: Thirty-seven male mountain bikers. INTERVENTIONS: None. MAIN OUTCOME MEASURES: Prerace and postrace, body mass, percent body fat, skeletal muscle mass, and selected hematologic and urinary parameters were measured to quantify changes in body mass and hydration status. Athletes reported their fluid intake during the race. RESULTS: The athletes lost 1.4 kg in body mass (P < 0.001), equal to 1.9% body mass. Fat mass remained stable and skeletal muscle mass decreased by 0.4 kg (P < 0.05). Prerace fat mass was correlated to total race time (r = 0.37, P < 0.05). The cyclists drank 6.5 (1.8) L of fluids during the race corresponding to 0.7 (0.2) L per hour. A significant inverse relationship was seen between change in body mass and race time when controlled for change in skeletal mass and fluid intake during the race (P > 0.05). Plasma sodium decreased by 0.7% (P < 0.05), plasma volume increased by 1.4%, and plasma urea increased by 40% (P < 0.05). Urinary-specific gravity increased by 0.4% (P < 0.05). The decrease in skeletal muscle mass was associated with the increase in plasma urea (r = -0.34; P < 0.05). Fluid intake was associated with the change in urinary-specific gravity (r = -0.45; P < 0.01). CONCLUSIONS: We conclude that these mountain bike ultra-marathoners suffered a significant decrease in body mass and skeletal muscle mass but no dehydration.