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Tubulo-interstitial kidney disease is characterized by moderate proteinuria < 1 g/day of low molecular weight proteins in range of MW 10.000-50.000. Even in the physiological proteinuria of < 150 mg/day, tubulo-interstitial kidney disease may exist. Using optimized sodium dodecyl sulfate polyacrylamid gel electrophoresis (SDS-PAGE) according to the method of Melzer, even in proteinuria of less than 150 mg/day all relevant proteins for diagnosis of glomerular or tubulo-interstitial kidney disease can be detected. This study evaluates the tubulo-interstitial kidney disease due to polychemotherapy for different types of cancer in 115 children and in 16 children with pyelo-ureteral junction obstruction. Fifty-two and 63 children were followed up during and after chemotherapy, respectively. During therapy, renal damage was recorded in 43% of patients with leukemia, 56% with nephroblastoma, and 79% with other tumors. Tubular protein patterns were seen up to three years after termination of chemotherapy (25% in acute lymphoplastic leukemia, 35% in nephroblastoma and 62% in other tumors). Patients with persistent complete tubular proteinuria or mixed glomerular/tubular proteinuria were found to have a high risk for irreversible renal failure. Children with congenital pyelo-ureteral junction obstruction could also be classified according to SDS-PAGE protein patterns. Patients without parenchymal lesions did not need surgery. Most of those with pathologic findings in SDS-PAGE exhibited partial or complete remission after surgery. The highly discriminating SDS-PAGE permits a rapid, sensitive, reproducible, and reliable analysis of urine proteins for diagnosis and follow-up of all kinds of congenital or acquired renal parenchymal kidney diseases.