There is increasing evidence that mild dehydration plays a role in the development of various morbidities. In this review, the effects of hydration status on chronic diseases are categorized according to the strength of the evidence. Positive effects of maintenance of good hydration are shown for urolithiasis (category lb evidence); constipation, exercise asthma, hypertonic dehydration in the infant, and hyperglycemia in diabetic ketoacidosis (all category IIb evidence); urinary tract infections, hypertension, fatal coronary heart disease, venous thromboembolism, and cerebral infarct (all category III evidence); and bronchopulmonary disorders (category IV evidence). For bladder and colon cancer, the evidence is inconsistent.