This review deals with renal and urinary tract complications in tumor patients. Neoplastic infiltration of the kidney and glomerular nephropathies in neoplastic diseases are discussed. Tumor obstruction of the urinary tract and renal disease due to tumor metabolites are important, potentially reversible complications. Radiation nephritis and drug-induced renal disease can often be avoided with the aid of precautionary measures. Urogenital infections in tumor patients are frequent and often due to atypical microorganisms. Guidelines for dosage modifications of antineoplastic agents in patients with renal insufficiency are summarized. Renal failure in tumor patients deserves a careful diagnostic and therapeutic approach. The potential renal and urinary tract toxicities of the different therapeutic modalities must be considered when planning therapy for cancer patients.