The aim of this study was to investigate the effect of a bone marrow transplantation (BMT) on renal function in children. In a 5-year period, 142 children received a BMT at the Department of Pediatrics of the University Hospital Leiden. The study was performed retrospectively using the estimated glomerular filtration rate before and 1 year after BMT, and weekly measurements of serum creatinine during the first 3 months after BMT for assessment of renal function. Patient characteristics (sex, age, diagnosis), conditioning regimen, type of BMT, major complications (sepsis, veno-occlusive disease and graft-versus-host disease (GVHD)) and the use of nephrotoxic medication were listed. In the first 3 months after BMT 17 (12%) patients died, 13 from transplant-related complications other than renal failure and four from relapse of the disease. Forty-eight children (34%) had a period with acute renal insufficiency. A high pre-BMT serum creatinine, transplantation with either a non-HLA-identical related or a matched unrelated donor were risk factors for acute renal insufficiency after BMT. Sepsis and the use of intravenous vancomycin were risk factors for acute renal insufficiency only for patients with a high pre-BMT serum creatinine. GVHD seemed to have a beneficial effect on renal function of BMT recipients. One year after BMT a total of 35 (25%) patients had died, 16 from transplant-related complications and 19 from relapse of the disease; another 17 patients could not be evaluated. Twenty-five of 90 evaluable children (28%) had chronic renal insufficiency. Chronic renal insufficiency 1 year after BMT was correlated with a high serum creatinine in the first 3 months after BMT. None of the children of this retrospective study on renal function after BMT needed dialysis.