Fanconi's syndrome is an unusual cause of renal insufficiency in pediatric patients. Infantile cystinosis is one of the identifiable and treatable etiologies of Fanconi's syndrome. Early diagnosis of cystinosis permits institution of specific therapy with cysteamine. A 3-year-old girl presented with failure to thrive, polyuria, and polydipsia. She was found to have renal tubular defect with renal dysfunction and bilateral small contracted kidneys. A renal biopsy revealed extensive giant cell transformation of podocytes in the glomeruli with focal tubular atrophy and dilatation. However, no crystals were identified. Subsequent ophthalmoscopic examination revealed fine cystine crystals in the cornea and a diagnosis of cystinosis causing Fanconi's syndrome was made. Polykaryocytic transformation of visceral epithelial cells is an important diagnostic clue of nephropathic cystinosis and should be carefully looked for in renal biopsy from a child with Fanconi's syndrome and renal insufficiency.