Introduction: Pseudo-Bartter syndrome is characterized by hypokalemic, hypochloremic metabolic alkalosis without evidence of primary renal tubular pathology. In patients with cystic fibrosis, this clinical picture may occur as the first manifestation of the disease. In already diagnosed patients, recurrence of electrolyte abnormalities is expected. Case presentation: An 8-month-old female infant with cystic fibrosis was admitted to our ward with insufficient oral intake, frequent regurgitation, and 1-week history of weight Ioss. Laboratory tests revealed metabolic alkalosis (pH: 7.69, HCO3 : 43.2 mmol/L), hyponatremia (Na+: 124 mmol/L), hypokalemia (K+: 2.5 mmol/L), and hypochloremia (Oh 70 mmol/L). Low electrolyte concentrations were measured in her uríne (Na+: 8 mmol/I, K+: 6 mmol/I, CI-: 44 mmol/I). With parenteral fluíd therapy, blood gas values and ioníc imbalances gradually resolved, urine electrolyte levels normalized, and complete clinical ímprovement was observed. The infant was discharged in good general condition with recommendations for increased salt intake and close outpatient follow-up. Conclusion: Pseudo-Bartter syndrome should be suspected in patients with cystic fibrosis who present with feeding difficulties and poor weight gain. Age under 1 year and warm weather are risk factors for its development. Therefore, routine urine electrolyte monitoring during summer in infants over 3 months, along with increased fluid and salt intake when necessary before symptom onset, is recommended.  Kulcsszavak: cystic fibrosis, Pseudo-Bartter syndrome, hypochloraemia, metabolic alkalosis, infant