Chitotriosidase, a macrophage marker, which is extremely increased in plasma of Gaucher patients, was measured in patients with beta-thalassemia, an haematological disorder characterized by the genetic defect of beta-globin chains synthesis resulting in unproductive erythropoiesis and enormous expansion of the reticuloendothelial system. Plasma chitotriosidase was increased to a variable extent in 13 of 70 patients with beta-thalassemia major treated with the intense transfusion regimen and iron chelation therapy. It was normal in 22 and slightly elevated in 3 subjects with beta-thalassemia intermedia which were not transfused. The highest levels of plasma chitotriosidase, as high as in Gaucher patients, were found in 7 (10%) of the beta-thalassemia major patients which also had the highest degree of iron overload as judged by their serum ferritin level (> 3000 ng/ml), high SGPT level and elevated urinary iron excretion. To our knowledge, beta-thalassemia is hitherto the only disorder in which an increase of plasma chitotriosidase, comparable to that seen in Gaucher disease, may occur. The increase of plasma chitotriosidase activity in beta-thalassemia patients with high iron overload, could be related to an iron mediated damage to the lysosomal apparatus. In addition, similarly to Gaucher disease, the increased chitotriosidase production in beta-thalassemia might reflect macrophage activation probably related to the intracellular iron overload, storage of erythrocytes membrane break-down products and oxidation of excess alpha-hemoglobin subunits. Further studies are required to define the role of chitotriosidase evaluation to assess the efficacy of chelation therapy in reducing the macrophage activation due to intracellular iron overload in beta-thalassemia.