Objective: To evaluate prospectively serological markers at baseline and during treatment in patients with rheumatoid arthritis (RA) initiating rituximab treatment, following failure of antitumour necrosis factor (TNF)-α therapy. Methods: Patients with RA and healthy control subjects were recruited. Plasma complement (C)3, C4, rheumatoid factor (RF), anticitrullinated protein antibody (ACPA), immunoglobulin (Ig)M, A and G, disease activity scores (DAS) and therapeutic response were recorded at baseline and at 6, 12 and 18 months. Results: Patients (n = 35) had significantly higher C3 and C4 levels than controls (n = 30). At 12 months after initiation of rituximab, C3 and C4 levels were significantly lower in patients who responded to treatment, compared with nonresponders. There were direct correlations between C3 levels and DAS at 12 months in the study population as a whole, and between IgM levels and DAS in responding patients after 6, 12 and 18 months’ treatment. Conclusions: C3 and IgM levels may represent potentially useful serological markers of disease activity during rituximab treatment in patients with RA.