OBJECTIVES: Dengue fever is an arbovirosis expanding worldwide, for which hydration has been reported to reduce the risk of hospitalization. Our objective was to estimate the volume of hydration in Reunionese patients with dengue. METHODS: A prospective observational study included patients presenting with a ’dengue-like’ syndrome in ambulatory care. General practitioners recruited patients during consultation, and beverage consumption over the previous 24 hours was reported at two different times. Warning signs were defined according to the 2009 WHO guidelines. RESULTS: GPs included 174 patients from April to July 2019. Average oral hydration volume was 1863 mL and 1944 mL, at the 1st and 2nd medical consultations, respectively. Water was the most wide consumed liquid. Drinking at least 5 glasses of liquid was significantly associated with fewer clinical warning signs at the 1st medical consultation (p = 0.044). CONCLUSIONS: Sufficient hydration volume could prevent dengue warning signs. Further studies with standardized measurement of hydration would be needed.