Chitotriosidase is a fully active chitinase produced and secreted by activated phagocytes. Plasma chitotriosidase activity is a well-established marker of total disease burden in Gaucher disease that has proved useful in monitoring the response to both enzyme replacement and substrate reduction therapies in patients with Gaucher disease. Increased chitotriosidase plasma activity has also been observed in several other lysosomal and non lysosomal disorders. Cystinosis, a rare multisystemic lysosomal storage disease, is characterized by the intralysosomal accumulation of free cystine in many cell types including phagocytes. We here report on plasma chitotriosidase activity in a child with nephropathic cystinosis. Increased plasma chitotriosidase activity (481 nmol/h per ml; normal range 0-150 units) was found on diagnosis and prior to the initiation of oral cysteamine (Cystagon) treatment. Serial estimations of plasma chitotriosidase activity showed that it correlated with leukocyte cystine content and decreased to 100 nmol/h per ml following 14 months' treatment. This novel observation suggests that cystinosis should be included in the differential diagnosis of disorders associated with increased plasma chitotriosidase activity. Furthermore, it suggests that serial estimations of plasma chitotriosidase activity could be of value in monitoring the response to oral cysteamine treatment in cystinosis.