Glucosylceramide-laden tissue macrophages in Gaucher patients secrete large quantities of chitotriosidase and CC chemokine ligand 18 (CCL 18), resulting in markedly increased plasma levels. We have comparatively investigated the occurrence of both parameters in plasma and urine samples of Gaucher patients. Chitotriosidase was high in urine samples of some symptomatic patients, but elevations did not correlate with increased plasma concentrations. Urinary chitotriosidase was particularly high in a patient with severe kidney involvement and local storage cell infiltration. Urinary levels of CCL18 were also highly elevated in samples from Gaucher patients as compared to controls. The median value of the CCL18/creatinine ratio in urine samples of 31 Gaucher patients was 143.3 pg/μmol (range 32-551) and in those of 12 normal subjects was 4.1 pg/μmol (range 1.3-6.8). In sharp contrast to chitotriosidase, increases in the low-molecularmass chemokine CCL18 in urine and plasma specimens of Gaucher patients correlated well. A correlation was also observed for reductions in urinary and plasma CCL18 following therapy. It is concluded that assessment of urinary CCL18 of Gaucher patients gives insight into the total body burden on Gaucher cells, whereas that of chitotriosidase does not. Urinary chitotriosidase appears rather to be a reflection of renal pathology.