A soluble form of CD14 (sCD14) was assessed with an ELISA assay in the serum of the following three clinical groups: 35 patients with an inactive phase of systemic lupus erythematosus (SLE), 17 patients with SLE relapses, and 65 normal healthy volunteers. Increased levels of sCD14 were observed in all patients suffering from SLE compared with normal controls. In addition, patients with active SLE revealed higher serum concentrations of sCD14 (median 6.9 mg/l) than patients under remission (4.1 mg/l; P < 0.0001). Serum values of sCD14 correlated neither with the number of peripheral blood monocytes bearing the CD14 membrane antigen, nor with serum concentrations of IL-1 beta. Serum sCD14 was compared with other clinical parameters used to monitor the clinical course of patients with SLE, among them complement C3, anti-dsDNA antibodies and soluble IL-2 receptor (sIL-2R). A good correlation emerged between sCD14 and C3 as well as sIL-2R concentrations, but sCD14 and anti-dsDNA titres disclosed no significant correlation in both groups of patients with SLE. Serial studies in patients with severe SLE showed that serum sCD14 closely parallels the clinical course as defined by an activity score. Our data suggest that serum sCD14 represents a promising parameter to monitor disease activity in patients with SLE.