The knowledge of renal function in the course of BMT is poor. We prospectively investigated glomerular and tubular function in 42 children who underwent BMT because of malignancy. Seventeen children were transplanted autologously. Investigations were performed before and immediately after the conditioning regimen. Inulin and creatinine clearance, albuminuria, urine excretion of alpha 1-microglobulin, beta-N-acetylglucosaminidase, alanine-aminopeptidase, intestinal alkaline phosphatase, and Tamm-Horsfall-Protein as well as sodium- and phosphatreabsorption were measured. The patients were classified regarding use of total body irradiation (tTBI) in the conditioning regimen. BEFORE CR: Glomerular filtration rate (GFR) was not influenced by the underlying diagnosis or previous treatment. Mean GFR was elevated compared with the reference group. Microalbuminuria was elevated in 15% of patients, and mean levels were higher than in the reference group. Proximal tubular dysfunction was indicated by an elevated excretion of alpha 1-MG in 54%, of beta-NAG in 66%, of AAP in 40%, and of IAP in 47%. Fractional sodium excretion was abnormal in 21%, phosphate reabsorption in 5% and THP-excretion in 7% of the patients. AFTER CR: Creatinine clearance was not affected by CR. After CR alpha 1-MG, beta-NAG, FENa, AAP, and IAP were increased compared with values before CR. TP/Clcr was decreased. Excretion of THP was not altered by CR. In patients without fTBI there was a greater increase in alpha 1-MG excretion and decrease in phosphate reabsorption after CR compared with patients conditioned with fTBI. We conclude that significant proximal tubular dysfunction is present in about 50-60% of patients before and in nearly all alter CR. Distal tubular function was less severely affected. Severity of nephrotoxicity after CR did not correlate with pre-existing abnormalities.