Correspondence to: J.C. Korevaar, Department of Clinical Epidemiology & Biostatistics (J1B-209), Academic Medical Center, University of Amsterdam, P.O. Box 22660, 1100 DD Amsterdam, The Netherlands. j.korevaar@amc.uva.nl Received 4 November 2005; accepted 23 June 2006. a A.J. Apperloo, J.A. Bijlsma, M. Boekhout, W.H. Boer, H.R. Buller, F.Th. de Charro, C.W.H. de Fijter, C.J. Doorenbos, W.J. Fagel, G.W. Feith, L.A.M. Frenken, P.G.G. Gerlag, J.P.M.C. Gorgels, W. Grave, R.M. Huisman, K.J. Jager, K. Jie, W.A.H. Koning-Mulder, M.I. Koolen, T.K. Kremer Hovinga, A.T.J. Lavrijssen, A.J. Luik, K.J. Parlevliet, M.H.M. Raasveld, M.J.M. Schonck, M.M.J. Schuurmans, C.E.H. Siegert, C.A. Stegeman, P. Stevens, J.G.P. Thijssen, R.M. Valentijn, M. van Buren, M.A. van den Dorpel, P.J.M. van der Boog, J. van der Meulen, F.M. van der Sande, A. van Es, J.A.C.A. van Geelen, G.H. Vastenburg, C.A. Verburgh, H.H. Vincent, P.F. Vos. The Netherlands Cooperative Study on the Adequacy of Dialysis (NECOSAD), a large prospective cohort study, was started in 1997. A majority of Dutch dialysis centers participated in this study. The aims of NECOSAD were to monitor the quality and adequacy of dialysis treatment and to provide evidence for optimal treatment of dialysis patients. Within NECOSAD, a sizeable variation in clinical and treatment characteristics was observed. These differences were due partly to differences in case-mix but mainly to differences in treatment regimes between centers. Due to these differences in usual care, we were able to study similar patients who obtained a different treatment regime. At this moment, several important issues regarding optimal dialysis treatment have been studied: the effect of the start of dialysis treatment, adequate dialysis dose, and evaluation of international guidelines, with special attention to comparisons between hemodialysis (HD) and peritoneal dialysis (PD) patients. All end-stage renal disease (ESRD) patients included in NECOSAD were new on dialysis treatment and it had to be their first renal replacement therapy. Clinical and treatment characteristics and data on important patient outcomes, such as mortality, morbidity, and health-related quality of life (HRQOL) were collected at 6-month intervals. Patients were followed until death, transplantation, or refusal of the patient to continue participation. Patient inclusion and follow-up are, at this time, still going on. By August 2005, 1983 patients had been included; 71% started with HD and 29% with PD. Median followup time was 25 months (interquartile range: 12 – 45 months) and maximum follow-up was 8.5 years. Over 400 patients have a follow-up period of at least 4 years. So far, 763 (39%) patients have died, 494 (25%) have received a kidney transplant, and 10% have refused further participation. Two-year survival of all included patients is 75%; 2-year survival is 80% for patients starting with PD and 73% for patients starting with HD.