The present paper deals with the rare phenomenon of the sequential development of Waldenström’s macroglobulinemia (WM) and γ1 heavy chain disease (γ1-HCD) in a single patient. The coexistence of both monoclonal proteins lasted for more than 9 years. Serial determinations were made of serum levels of IgM, IgA, intact IgG, and the γ-HCD protein. Immunofluorescent studies of a bone marrow aspirate confirmed the diagnosis of an IgM(κ) monoclonal gammopathy and also showed cells which were thought to synthesize the γ-HCD protein. The molecular weight of the γ-HCD protein was 60,000 in the dimeric form. It is concluded that the original IgM(eκ) monoclonal gammopathy predisposed the patient for a second lymphoproliferative disorder (the γ-HCD) and that both monoclonal proteins are immunogenetically