We investigated whether ultraendurance runners in a 100-km run suffer a decrease of body mass and whether this loss consists of fat mass, skeletal muscle mass, or total body water. Male ultrarunners were measured pre- and postrace to determine body mass, fat mass, and skeletal muscle mass by using the anthropometric method. In addition, bioelectrical impedance analysis was used to determine total body water, and urinary (urinary specific gravity) and hematological parameters (hematocrit and plasma sodium) were measured in order to determine hydration status. Body mass decreased by 1.6 kg (p less than 0.01), fat mass by 0.4 kg (p less than 0.01), and skeletal muscle mass by 0.7 kg (p less than 0.01), whereas total body water increased by 0.8 L (p less than 0.05). Hematocrit and plasma sodium decreased significantly (p less than 0.01), whereas plasma urea and urinary specific gravity (USG) increased significantly (p less than 0.01). The decrease of 2.2% body mass and a USG of 1.020 refer to a minimal dehydration. Our athletes seem to have been relatively overhydrated (increase in total body water and plasma sodium) and dehydrated (decrease in body mass and increase in USG) during the race, as evidenced by the increased total body water and the fact that plasma sodium and hematocrit were lower postrace than prerace. The change of body mass was associated with the change of total body water (p less than 0.05), and we presume the development of edema. (Contains 2 tables and 4 figures.)