A loss in total body mass during an ultraendurance performance is usually attributed to dehydration. We identified the changes in total body mass, fat mass, skeletal muscle mass, and selected markers of hydration status in 31 male nonprofessional ultratriathletes participating in a Triple Iron triathlon involving 11.4 km swimming, 540 km cycling and 126.6 km running. Measurements were taken prior to starting the race and after arrival at the finish line. Total body mass decreased by 1.66 kg (SD = 1.92; -5.3 kg to +1.2 kg; p less than 0.001), skeletal muscle mass by 1.00 kg (SD = 0.90; -2.54 kg to +2.07 kg; p less than 0.001), and fat mass by 0.58 kg (SD = 0.78; -1.74 kg to +0.87 kg; p less than 0.001). The decrease in total body mass was associated with the decrease in skeletal muscle mass (r = 0.44; p less than 0.05) and fat mass (r = 0.51; p less than 0.05). Total body water and urinary specific gravity did not significantly change. Plasma urea increased significantly (p less than 0.001); the decrease in skeletal muscle mass and the increase in plasma urea were associated (r = 0.39; p less than 0.05). We conclude that completing a Triple Iron triathlon leads to decreased total body mass due to reduced fat mass and skeletal muscle mass but not to dehydration. The association of decrease in skeletal muscle mass and increased plasma urea suggests a loss in skeletal muscle mass. (Contains 3 tables and 4 figures.)j