The aim is to study the osmoregulatory renal function as a key mechanism for cell volume regulation in patients with cystic fibrosis (CF). Materials and methods of research: 59 children were examined (CF, n=24; pneumonia, n=19; healthy children, n=15). Osmoregulatory function was assessed after fluid restriction during the night and water load of 10 ml per kg of body weight. Results: CF patients showed a positive correlation between the renal clearances of solute free water and sodium free water in night urine specimens. After water loading, similar renal reaction was found in all groups of examined children, which confirmed the assumption about the changes in regulation of renal function in patients with CF. A provision on increasing the sensitivity of the osmoregulation system to maintain cell volume in children with CF is justified.