In order to investigate, if complement levels can be used as an indicator of clinical activity in systemic lupus erythematosus (SLE), levels of C3, C4, CH50, and C3d were measured in 79 patients, 41 with inactive, 31 with moderately active and 7 with severely active disease. Our study shows that C3d, and particularly the C3d/C3 ratio, provide sensitive markers for disease activity in SLE. Since C3d is a direct measurement of complement turnover, it reflects complement activation better than C3, C4 and CH50.