The 'primary' form of chronic cold agglutinin disease is a clonal B-cell lymphoproliferative disorder that is notoriously difficult to treat with drugs, including corticosteroids, alkylating agents, alpha-interferon and purine analogues. We performed a small, open, uncontrolled, prospective study to evaluate the effect of therapy with the monoclonal anti-CD20 antibody rituximab. Six patients with clonal CD20(+)kappa(+) B-cell proliferation received seven courses of rituximab 375 mg/m(2), d 1, 8, 15, and 22. One patient achieved a complete response. Four partial responses were observed, including a response to re-treatment in one patient. Two patients were categorized as non-responders. Haemoglobin levels increased by a median of 4.1 g/dl in the total group and 4.7 g/dl in the responders, who also experienced a substantial improvement of clinical symptoms. The treatment was well tolerated. We discuss the effect of rituximab therapy compared with other treatment options, and try to explain why two individual patients did not respond. Despite the small numbers, the results are very encouraging. Further studies of rituximab therapy for chronic cold agglutinin disease are warranted.