Nephropathic cystinosis is a rare genetic disease with early renal affection and progression to end stage renal failure if not treated with cysteamine. This study aims to evaluate the activity of chitotriosidase enzyme in patients with nephropathic cystinosis and suggesting its role as a marker in specific diagnosis and follow up of these patients. Fifteen Egyptian patients with nephropathic cystinosis and forty six control subjects were studied and results suggested that chitotriosidase enzyme show significant higher values in patients with nephropathic cystinosis(median value 467nmol/ml/h.) with noted relation between the enzyme level and age, dose and duration of cysteamine therapy and presence of renal impairment.