Complement activation products (CAP) have been reported as sensitive markers of disease activity in systemic lupus erythematosus (SLE). We have measured seven parameters of the complement system (C3, C4, factor B, C3a, C4a, iC3b, and the terminal complement complex-TCC-) in 61 SLE patients to study their interrelationship and relative efficacy as diagnostic indicators of lupus activity. Disease activity was judged according to a clinical index (SLEDAI) to be active in 22 and inactive in 39 patients. Subjects with active SLE showed increased levels of C3a, C4a, and TCC compared with those of stable lupus and normal controls, and plasma concentrations of these CAP manifested a positive correlation with disease activity scores. However, values of factor B and iC3b did not correlate with lupus flares. Serum C3 levels were a better reflection of the degree of SLE activity than were C4 levels. The anaphylatoxins were extremely sensitive markers of disease activity but they lacked enough specificity, and iC3b was not at all informative for this purpose. On the whole, TCC concentration was the most useful parameter (77% sensitivity, 80% specificity) to monitor lupus activity, correlating the best with the activity scoring system, and thus offers a better laboratory marker of lupus severity than conventional measurements of complement.