Abstract: It is shown that a defect in the chitotriosidase gene is a risk factor with respect to susceptibility for infectious diseases with chitin-containing pathogens and the development of (rheumatoid) arthritis. The molecular basis of the relatively common chitotriosidase deficiency is a 24 bp duplication in the chitotriosidase gene. A convenient method allowing analysis of chitotriosidase genotype and subsequent determination of increased risk has been developed. Chitotriosidase is shown to be selectively secreted by macrophages upon specific activation and excreted by neutrophils by release of specific granules upon an appropriate stimulus. It has been shown that the measurement of plasma chitotriosidase activity can be successfully used for diagnosis of specific disorders and monitoring of efficacy of therapeutic interventions, at least in combination with information on the chitotriosidase genotype status of an individual.