Cystinosis is the major cause of inherited Fanconi syndrome, and should be suspected in young children with a failure to thrive and with signs of renal proximal tubular damage. This is a study of a case report of cystinosis revealed by a growth failure and chronic dehydration. A 9-month-old boy was referred to our department for evaluation of polyuria and polydipsia. Clinical examination showed dehydration and enlarged wrists and rachitic rosaries. The presence of metabolic acidosis, hypokalemia, hypochloremia with proteinuria, polyuria, and hypercalciuria was suggestive of inherited Fanconi syndrome. The diagnosis of cystinosis was confirmed by an increased leukocyte cystine level.