Background:The assessment of proper hydration status in hemodialysis patients is difficult. None of the currently available markers or measures is clinically relevant. Recently, a human pre-pro-vasopressin (1-164) split product (copeptin) - a new surrogate marker of hydration status - was introduced. The aim of the study was to analyze body weight changes in the early posttransplant period in relation to plasma copeptin (CPP) levels before kidney transplantation. Methods:Plasma CPP and N-terminal prohormone for brain natriuretic peptide (NT-proBNP) concentrations and osmolality were measured in 130 kidney recipients directly prior to transplantation and, additionally, in 78 of them at the 14th day posttransplant. Hydration status at transplantation was calculated from the difference in patient's body weight before transplantation and at the discharge. Results:During the posttransplant hospitalization, the average weight change was minus 1.6 kg, varying from 10.5 kg loss to weight gain of 5 kg. The overall weight loss was significantly related to pretransplant serum concentration of CPP (r=0.238), but not of NT-proBNP or osmolality. Patients with the lowest initial CPP level (first tertile) had smaller posttransplant weight loss. The multivariate regression model revealed that the variability of posttransplant weight loss is explained by the number of antihypertensive drugs used prior to transplantation [β=0.213 (0.049-0.377)] and pretransplant CPP values [β=0.233 (0.069-0.397)]. Conclusions: Elevated serum copeptin level predicts a rapid weight loss after kidney transplantation and seems to characterize the subgroup of patients with greatest overhydration. These results suggest the dysregulation of physiological mechanisms of copeptin secretion in hemodialysis patients.