A recent study has shown that chitotriosidase (Chit) may play a role in the pathogenesis of multiple sclerosis (MS). Plasma Chit activity was investigated in 219 untreated MS patients and 160 healthy controls (HC) by means of a fluorometric enzyme activity assay. Chit activity was also measured in a subgroup of 46 patients following treatment with interferon-beta (IFNbeta). Overall, plasma Chit activity was significantly increased in MS patients compared with HC, but no differences were observed between relapsing and progressive clinical forms. In addition, Chit activity was similar between patients during relapse and patients during clinical remission. Treatment with IFNbeta was associated with a significant increase in Chit activity compared with untreated patients in both responders and non-responders to treatment. Although these findings suggest a role of Chit in MS, our data do not support an association between plasma Chit activity and MS clinical course and clinical response to IFNbeta treatment.