Acute gastroenteritis is common in childhood. The estimation of the degree of dehydration is essential for management of acute gastroenteritis. Plasma water was assessed as a diagnostic tool in children with acute gastroenteritis and dehydration admitted to hospital. In a prospective cohort study, 101 patients presenting at the emergency department with dehydration were included. Clinical assessment, routine laboratory tests, and plasma water measurement were performed. Plasma water was measured as a percentage of water content using dry weight method. During admission, patients were rehydrated in 12 h. Weight gain at the end of the rehydration period and 2 weeks thereafter was used to determine the percentage of weight loss as a gold standard for the severity of dehydration. Clinical assessment of dehydration was not significantly associated with the percentage of weight loss. Blood urea nitrogen (r = 0.3, p = 0.03), base excess (r =-0.31, p = 0.03), and serum bicarbonate (r = 0.32, p = 0.02) were significantly correlated with the percentage of weight loss. Plasma water did not correlate with the percentage of weight loss. On the basis of the presented data, plasma water should not be used as a diagnostic tool in the assessment of dehydration in children with acute gastroenteritis.