Hyponatremia is not rare in cystic fibrosis and might be due to several mechanisms. An endocrine and renal imbalance in water and salt homeostasis was suggested. To address this hypothesis, we assessed the urinary concentrating and diluting ability in 12 cystic fibrosis patients (6 females, 6 males) and in two control groups: 14 children with pneumonia (9 females, 5 males) and in 13 healthy children (9 females, 4 males). Renal concentrating ability was evaluated following overnight water deprivation. Urine osmolality was not significantly different between groups. Renal diluting ability was assessed by means of a water-load test. This provoked a decrease in urine osmolality, as well as an increase in diuresis and solute-free water excretion. These changes were comparable among groups.Conclusion: Children with cystic fibrosis show a preserved renal concentrating and diluting capacity. A generalized endocrine and renal imbalance in water and salt homeostasis therefore appears unlikely.What is Known:•Hyponatremia sometimes occurs in cystic fibrosis.What is New:•Osmoregulation is normal in cystic fibrosis.