Objective. To determine the value of measurement of serum soluble tumor necrosis factor receptor (sTNFR), compared with established parameters such as anti–double‐stranded DNA, in monitoring systemic lupus erythematosus (SLE) disease activity, and to determine whether serum sTNFR are bioactive and can effectively inhibit TNF bioactivity. Methods. Fifty‐three consecutive ambulatory or hospitalized SLE patients and 140 consecutive healthy subjects were enrolled in a prospective cohort study. Serum levels of sTNFR were measured by a unique 2‐sided capture enzyme‐linked immunosorbent assay using mouse monoclonal antibodies and rabbit antisera against the sTNFR. Results. The mean ± SD concentrations of both the p55 (type I) and p75 (type II) soluble receptors were significantly higher in a group of 46 SLE patients than in controls: 1.89 ± 0.89 ng/ml versus 0.77 ± 0.19 ng/ml and 7.25 ± 3.89 ng/ml versus 3.02 ± 0.57 ng/ml, respectively (P < 0.0001 for both). The incidence and the extent of the increase among the healthy subjects and these patients (as well as in 7 additional patients on whom sequential studies were performed) correlated with disease activity more than did the occurrence of serum anti‐DNA antibodies (correlation coefficients with disease activity 0.81 and 0.85 for p55 and p75 sTNFR, respectively, and 0.51 for anti‐DNA antibodies). The increase in sTNFR levels seems to reflect, largely, enhanced formation, and only to a minor extent, reduced clearance due to impairment of renal function. Sera of the SLE patients had a marked inhibitory effect on the in vitro cytocidal activity of TNF, and this was shown to result entirely from their higher sTNFR receptor concentration. Conclusion. An increase in serum levels of sTNFR may become a useful marker for SLE activity since it shows a stronger correlation than do any other laboratory or clinical parameters employed presently in the daily clinical setting. At the concentrations attained in the serum of SLE patients, sTNFR effectively inhibit the bioactivity of TNF and may thus be a significant determinant of the intensity of the manifestations of SLE.