Detecting and treating dehydration in hospitalized patients is critical because of the adverse outcomes associated with this condition. Using a case-control design, this study estimated the incidence, risk factors, and outcomes of dehydration in hospitalized adults. The overall incidence rate for developing one of three ICD-9 codes for dehydration during a hospital stay was 3.5%. Cases and controls differed significantly on a number of clinical variables on admission; a large percentage of patients may have had dehydration on admission to the hospital. Mortality rates at 30 and 180 days postdischarge were significantly higher when dehydration was present. Patients may be discharged to rehabilitation settings in a dehydrated state, which prolongs recovery. Despite the increased risk for dehydration and higher rates of hospitalization in older populations, little systematic research has addressed the risk factors for and indicators of dehydration in hospitalized patients.