We have evaluate prospectively renal function in children receiving allogeneic Bone Marrow Transplantation (BMT) for hematological or metabolical diseases. We report our first evaluation in 33 patients (pts) who underwent BMT between oct '86 to oct '92. Renal functions tests included measurement of glomerular filtration rate (GFR), renal plasma flow (RPF), clearance reabsorption of Na, K, Cl, Ca and P. Tests were done pregraft and post graft. During BMT period mild renal disorders were observed in 13 pts: HTA (n = 4), increase of creatininemia (n = 8); more severe in 10 pts: acute renal failure (n = 8), hypokaliemia (n = 1), tubular dysfunction (n = 1). After BMT there was a significant decrease in GRF and RPF at 12 m and 24 m with a tendancy towards reascension of GRF and RPF at 36 m. The filtration fraction (FF) is not affected. A significant decrease of Ccr at 12 m, TR Na and TR Cl at 12 and 24 m, TR K at 24 m compared to To is observed. GRF and RPF at 12 m were not significantly influence by chemotherapy prior to BMT, Total Body Irradiation or Graft Versus Host Disease. Severe renal disorders after BMT influence significantly the evolution of GFR. Conclusion: A precise evaluation of renal functions is particularly important before and during transplantation to manage nephrotoxic drugs employed during this period.