Introduction. Infants with cystic fibrosis may fail to thrive despite recommended caloric intake because of electrolyte disurbances caused by salt depletion resulting in hypochloremic metabolic alkalosis or pseudo-Bartter's syndrome. In most patients reported symptoms began in infancy, but it may be an initial presentation of disease in a previously healthy adolescent. Case report. A 15-year-old boy was admitted for evaluation of recurrent episodes of malaise associated with dehydration and acute renal insufficiency. Laboratory analysis showed hypochloremic metabolic alkalosis with hyponatremia and hypokalemia. On admission the boy was obese, with body weight of 95.5 kg (> P97), height 174 cm (> P75), and body mass index of 31.2 kg/m2 (> P95). Physical examination was inconclusive. Blood pressure holter monitoring proved significant systolic hypertension. Routine urinalysis, protein and electrolyte levels in urine were normal. Plasma renin and aldosteron were normal. Sweat chloride concentration was 63 mmol/L. Genetic testing confirmed the diagnosis of cystic fibrosis. Conclusion. To our knowledge, this is the first reported case of atypical presentation of cystic fibrosis in an adolescent presented with pseudo-Bartter's syndrome and signs of obesity and hypertension. We suggest that every patient with hypochloremic metabolic alkalosis should be evaluated for cystic fibrosis.