The authors report on three infants with cystic fibrosis (CF), with different genotypes, presenting with different clinical manifestations, but having similar abnormal serum electrolytes (i.e. hyponatremia, severe hypochloremia and metabolic alkalosis). Despite the diagnostic investigations, the child who presents with severe electrolyte imbalance especially persistent hypochloremia and a family history of early infant death with respiratory or gastrointestinal problems should point to a diagnosis of CF Early identification and treatment remain critical to effective management. The diagnostic tool used, especially the sweat test, is needed for diagnostic investigations in Thailand.