The non-random association of Gaucher disease with polyclonal and monoclonal gammopathy has been known since 1950. The effect of treatment on monoclonal gammopathy is not well documented. We report on the long-term evolution of a biclonal gammopathy in a patient with type I Gaucher disease who was treated with splenectomy and enzyme replacement therapy. A 44-year-old man presented with hepatomegaly and massive splenomegaly. Bone marrow aspirate contained typical Gaucher cells and beta-glucosidase was low in peripheral blood leukocytes. Mutations N370S and R120W were detected. Serum protein electrophoresis disclosed two spikes in gammaglobulins. Immunofixation identified two monoclonal components: IgG kappa and IgA kappa. Gammaglobulin concentration was 31.6 g/L. A splenectomy was performed on September 2003 because of massive splenomegaly (9500 g). Two months after the splenectomy, gammaglobulin concentration was 25.2 g/L. Enzyme replacement therapy (Cerezyme 45 UI/kg every two weeks) was prescribed from April 2004 because of significant hepatomegaly and cholestasis. In April 2007 (3 years after the beginning of treatment), serum electrophoresis showed the persistence of two spikes with gammaglobulin concentration at 20.5 g/L. Simultaneously, chitotriosidase activity decreased from 6181 to 2877 nkat/L. Our observation and previous reports suggest that enzyme replacement therapy is more effective in polyclonal hypergammaglobulinaemia than in monoclonal gammopathy.