Fumonisins are mycotoxins produced worldwide by Fusarium fungi, principally F. moniliforme. The fungus is present in virtually all harvested corn, but the toxins produced are variable. The toxins, especially fumonisin B1, cause mild to fatal diseases in animals, with peculiar species specificity for the dominant signs of toxicity. The mechanism of toxicity is poorly understood, but it appears to be related to interference with sphingolipid biosynthesis in multiple organs. Whereas brain, lung, and liver are well-known target organs, toxic effects on the kidney are also widespread and have only recently begun to be characterized. Increased urine volume and decreased osmolarity are early changes associated with the toxin, as are increased excretions of high- and low-molecular-weight proteins. Enzymuria in vivo, reduced ion transport in vitro, and elevation of free sphinganine in renal tissue and in urine are present. An increase in serum creatinine and blood urea nitrogen and histopathologic change in renal tubules occur later and at higher doses. The morphologic change principally affects the junction of cortex and medulla and includes prominent apoptosis of epithelial cells of proximal convoluted tubules. Nephrotoxicity has been reported in several species, and in rats and rabbits, the kidney appears to be the most sensitive target organ.