Objective. To study the association between disease activity and complement activation prospectively in patients with systemic lupus erythematosus (SLE).

Patients and methods. Twenty-one SLE patients were examined monthly for 1 yr. Disease activity, autoantibodies, conventional complement tests and the following complement activation products were investigated: C1rs–C1inh complexes, C4bc, Bb, C3a, C3bc, C5a and the terminal SC5b–9 complement complex (TCC).

Results. Modest variation in disease activity was noted. None of the patients had nephritis. Flare was observed at 27 visits. Four patients had anti-C1q antibodies in conjunction with modestly low C1q concentrations. The complement parameters were rather constant during the observation period. Slightly to moderately decreased C4 (0.05–0.10 g/l) was found in 10 patients and severely decreased C4 (<0.05 g/l) in seven patients. Decreased C4 was not associated with increased complement activation. Complement activation products were either normal or slightly elevated. TCC was the only activation product correlating significantly with score for disease activity at flare. None of the variables tested predicted flares.

Conclusion. Complement tests are of limited importance in routine examination of SLE without nephritis, although TCC is suggested to be one of the most sensitive markers for disease activity.