Background: Atherosclerosis is considered to be an inflammatory disease in which the initial process is augmented infiltration of monocytes into the vessel wall and their subsequent differentiation from macrophages into lipid-laden foam cells. Chitotriosidase is one of the most quantitative proteins secreted by activated macrophages, so the aim of this study was to investigate the association of the level of serum chitotriosidase activity with atherosclerotic coronary artery disease (CAD). Methods and Results: A total of 200 subjects undergoing coronary angiography were divided into 4 subgroups according to the number of diseased vessels and their serum chitotriosidase activity levels were measured. Serum chitotriosidase activity in patients with CAD was significantly higher than in normal control subjects (p<0.001). Serum chitotriosidase activity was also significantly associated with the extent of CAD as defined by the number of stenosed vessels (p<0.001). Conclusion: Serum chitotriosidase activity can be considered a strong inflammatory marker of CAD. Moreover, plasma chitotriosidase activity may be also regarded as a quantitative indicator of disease extent, as well as being a marker of disease presence.