Abstract: Physicians recommend a high water intake to prevent cerebral infarction by decreasing blood viscosity. However, there is no evidence that high water intake decreases viscosity, although it increases urinary frequency. Therefore, we investigated the change of blood viscosity and urinary frequency by high water intake. We examined the change of blood viscosity after acute water intake, the circadian rhythm of blood viscosity, and the change of early morning blood viscosity and urinary frequency after high water intake for 1 week in healthy volunteers aged 31–75 years. Blood viscosity decreased transiently by acute water intake, but remained within the range of its circadian rhythm. Morning blood viscosity did not alter by high water intake for 1 week, although urinary frequency increased. Because excessive water intake increases urinary frequency without decreasing blood viscosity, a high water intake should not be recommended in persons with urinary frequency.