Objective. Serum CH50 and C4 levels are usually normal or elevated in rheumatoid arthritis (RA) but are classically decreased in patients with serious extra-articular manifestations (SEAMs) of the disease. The objective of this study was to evaluate whether complement assays are useful in diagnosing or predicting SEAMs of RA. Methods. First, a cross-sectional study of 405 patients admitted for RA compared patients with and without hypocomplementemia. Then, a retrospective longitudinal design was used to investigate within-patient complement level variations over time. Results. In the univariate analysis, patients with low CH50 and C4 levels were more likely to have vasculitis and/or cryoglobulinemia than those with normal CH50 and C4 levels, and nodules were more common in the patients with low than with normal C4 levels. In a multivariate model based on symptoms, low C4 was associated with vasculitis and pleurisy and low CH50 with vasculitis. However, these associations were too weak to make CH50 and C4 determination useful for detecting SEAMs, and the within-subject variations in patients with SEAMs limited the predictive value of these assays. Conclusion. Hypocomplementemia is of limited usefulness for detecting or predicting SEAMs.