The Modification of Diet in Renal Disease (MDRD) equation is widely used for the estimation of glomerular filtration rate (GFR) from plasma creatinine. It has been well validated in patients with various degrees of impaired kidney function, but not in patients with end-stage renal disease (ESRD). Plasma creatinine is determined by GFR and muscle mass. Importance of the latter may increase at low GFR. Our aim was to firstly compare estimated GFR (eGFR by MDRD equation) with measured GFR (mGFR, mean of creatinine and urea clearance) just before the start of dialysis. Secondly, the relationship of eGFR and mGFR with mortality and muscle mass was analysed. ESRD patients with 24-h urine collections and a plasma sample available at the start of dialysis [n = 569, 61% male, mean (standard deviation) age 58 (15) years] were selected from the Netherlands Cooperative Study on the Adequacy of Dialysis. Incident dialysis patients were followed until death, transplantation or end of study. mGFR was 6.0 (2.6) and eGFR was 6.8 (2.4) mL/min/1.73 m(2). Although eGFR overestimated mGFR with only 0.8 mL/min/1.73 m(2), limits of agreement ranged from - 4.1 to + 5.6 mL/min/1.73 m(2). The highest eGFR values were associated with the highest mortality rates [adjusted hazard ratio 1.4 (1.0, 1.9)]. eGFR but not mGFR was associated with muscle mass (P = 0.001). These data imply that estimation of GFR by equations using plasma creatinine in the denominator cannot be used for this purpose in patients with ESRD because the effect of GFR on plasma creatinine is overruled by that of muscle mass.