Data on free water excretion capacity of renal transplant recipients are scant. The aim of this study was to evaluate the ability of electrolyte free water clearance (E-CH(2)O) by the allograft in renal transplant patients and the effects of various immunosuppressive drugs. Renal transplant recipients with good graft function (creatinine < 1.5 mg/dL) as well as controls were divided into five groups according to their immunosuppressive regimen: group 1, azathioprine (n = 1.5); group II, cyclosporine (n = 28); group III, tacrolimus (n = 28); group IV healthy controls (n = 20); and group V renal transplant donors (n = 16). Following a 12-hour fast, we administered oral water loading (20 mL/kg) with urine collection for 3 hours. We calculated creatinine clearance for 3 hours and E-CH(2)O. No matter which immunosuppressive drug, the E-CH(2)O of recipients (groups I, II, and III) was lower than that of donors or healthy controls. The creatinine clearance of the cyclosporine arm was significantly lower than all of the other groups. Decreased E-CH(2)O in renal transplant patients might be due to diminished water input to the loop of Henle related to subclinical allograft insufficiency as a result of posttransplantation pathology and/or immunosuppressive drug therapy or the transport of water into the extrarenal interstitium as a result of vascular endothelial dysfunction due to the pretransplant uremic milleu.