Renal function after orthotopic liver transplantation (OTL) is very frequently reduced and its level exerts a significant effect on the morbidity and mortality of these subjects. One of the main factors with a negative impact on renal function after OTL is the nephrotoxic action of cyclosporin A (CsA). Renal function after OTL is usually evaluated on the basis of glomerular filtration (GF). As chronic nephrotoxicity of CsA is manifested in the histological picture by significant tubulointerstitial affection, in 75 subjects after OTL the spontaneous concentrating and acidifying capacity of the kidneys was investigated. The value of urine osmolality (UOSM) assessed after noctunal withdrawal of fluids was in 72.7% lower than in healthy subjects and did not reach 600 mOsm/kg H2O, although the serum creatinine concentration (Scr) was still within the normal range. The pH value of the morning urine did not reach in 38.2% the required value of 6.0 although Scr was within the normal range. Between values of UOSM after nocturnal liquid withdrawal and GF assessed on the basis of inulin clearance (Cin) was a significant direct relationship, however the scatter of values was considerable (r = 0.226, p < 0.05). Between pH values of the morning urine and Cin no correlation was found. The assembled results support the idea that the concentrating activity of the kidneys in subjects after OTL treated with CsA is reduced. This reduced concentrating capacity is already apparent on the basis of UOSM of morning urine after nocturnal fluid withdrawal. Although this defect is also frequent in subjects with a normal Scr value, the authors assume that the use of this simple evaluation of the concentrating capacity (it does not burden the patient nor the attending staff) could be useful in the early diagnosis of tubulointerstitial affection.