Cystic fibrosis (CF) is an autosomal recessive disease affecting exocrine glands and frequently presenting with respiratory and gastrointestinal symptoms. Acute dehydration with hypokalemia and metabolic alkalosis is a well-known clinical form of CF reported in infancy but it is not reported in older children. A healthy 5-year-old boy with no previous respiratory and gastrointestinal symptoms but recurrent hypokalemic dehydration and metabolic alkalosis is presented. Repeated sweat tests performed on two occasions revealed high sweat chloride concentrations confirming the diagnosis of CF.