Recently, soluble CD163 (sCD163) has been identified as a macrophage/monocyte-specific plasma protein and increased concentrations have been measured in patients with infection and myeloid leukaemia. In the present study we investigated the levels of sCD163 in patients with Gaucher's disease, an inherited lysosomal storage disorder characterised by hepato- and splenomegaly due to excessive accumulation of macrophages. The sCD163 plasma levels, median (25-75 percentiles), were far above the levels in normal subjects [7.1 mg/L (4.8-10.3) vs. 1.9 mg/L (1.5-2.4), P < 0.0001]. After initiation of enzyme supplementation therapy, the sCD163 levels were significantly reduced [4.7 mg/L (3.2-6.6), P = 0.0004]. sCD163 correlated with disease severity (rho = 0.43, P < 0.0061) and chitotriosidase activity (rho = 0.71, P > 0.0001). This study further establishes that sCD163 may be a valuable laboratory parameter in monitoring disease with increased macrophage activity.