In this study, the danger of dehydration and heatstroke in a walking event in the summer was examined. Subjects were 64 males, 11 females, and the average age was 62.3±7.8 years old. The following were measured before and after 25 km walking : body weight, blood pressure, heart rate, tympanic temperature, and fluid intake. WBGT in the walking event was 22.6℃ mean value. Time of the 25 km walking was 5 hours 45 minutes, and the number of steps were 36,570±2,741 steps. The body weight loss by the walking was 1.36±0.57 kg, 2.24±0.96%. This body weight loss rate increased with the age. The fluid intake was 1,327±608 g, and sweat loss was 2,621±740 g in the walking. The tympanic temperature after walking was 38.02±0.60℃, and increased 0.99±0.67℃ from before walking. In the walking of middle-aged and elderly persons in the summer, there is danger of dehydration and heatstroke, therefore enough water and salt intakes are necessary.