Juvenile idiopathic arthritis (JIA) is an inflammatory joint disease of unknown etiology. The pathogenesis is driven by T and B cells. The role of macrophages remains unclear. Chitotriosidase belongs to the chitinase protein family and is secreted by activated macrophages. The chitinases are able to catalyze the hydrolysis of chitin or chitin-like substrates such as 4-methylumbelliferyl chitotrioside. Chitotriosidase activity was determined using the substrate 4-methylumbelliferyl beta-DNN'N''-triacetylchitotrioside (4-MU-TCT, SIGMA Chemical Co.). The substrate and serum were incubated with the serum in a citrate/phosphate buffer. The reaction was stopped by adding a buffer (Na2CO3). The fluorescence of 4-methylumbelliferone was evaluated by fluorimeter at excitation 360 nm and emission 450 nm. We report about chitotriosidase measurements in patients with JIA. The chitotriosidase level in synovial fluid was up to approximately 1,000 nmol/(h ml) at disease onset before therapy. The level in the sera was below 600 nmol/(h ml). Serum chitotriosidase levels could represent the activity of macrophages in the synovial fluid in JIA.