Objectives: The aim of the present study was to establish the range of chitotriosidase (CT) activity in normal individuals, patients with Gaucher disease (GD) and Niemann–Pick disease (NPD), types A or B. The kinetics of CT in these three groups was also investigated. Design and methods: CT activity, as well as Km, Vmax, optimum pH, and thermal stability of the enzyme were determined in the plasma of control, GD, and NPD subjects. Results: CT activity in GD and NPD patients was, respectively, around 600-fold and 30-fold greater than in normal individuals. We observed significant differences in optimum pH, Vmax, and thermal stability between the various groups. Km was different in normal individuals relative to GD and NPD patients. However, there was no significant difference between Km values in patients with GD and with NPD. Conclusions: Based on the differences found in the biochemical parameters studied, our results may be important to help the identification of patients not only with GD but also with NPD. D 2004 The Canadian Society of Clinical Chemists. All rights reserved.