Renal handling of sodium, potassium, calcium and phosphorus was investigated in 9 patients with cystic fibrosis (CF). Both under baseline conditions and during saline-induced diuresis, urinary excretion rate of sodium (UNaV), potassium, phosphorus and calcium did not differ significantly from control values although the fractional excretion of sodium was significantly higher in patients with CF during saline diuresis. When distally acting diuretics were administered, this difference between CF patients and controls was magnified and in addition, UNaV in CF patients was also significantly higher than in controls. Following saline loading the increment in glomerular filtration rate (GFR) in patients with CF was significantly lower than that of controls. We conclude that patients with CF when subjected to volume expansion demonstrate a lower tubular reabsorptive capacity of sodium as well as a reduced ability to increase their GFR. The defect in sodium reabsorption is probably located in the proximal tubule.