Consistent with previous reports1–2 of children with metabolic alkalosis and cystic fibrosis, the molar urinary chloride/creatinine ratio was low in our pediatric patient, while it was normal or elevated in our adult patients. If metabolic alkalosis in our adult patients was the result of respiratory alkalosis superimposed on respiratory acidosis (posthypercapnic metabolic alkalosis), urinary chloride excretion would be low. Inability to conserve chloride in the urine during metabolic alkalosis is associated with hyperaldosteronism, hypokalemia, refeeding syndrome, and diuretic therapy. Aldosterone levels were low in all episodes involving adult patients. Hypokalemia and refeeding syndrome were not present. None of the patients was receiving diuretic therapy. Alternative explanations for the lack of appropriate urinary chloride retention in adults with cystic fibrosis and metabolic alkalosis, such as an age-related change in renal tubular function or the renal expression of defective cystic fibrosis transmembrane conductance regulator, have not been described.