Philadelphia chromosome-positive chronic myeloid leukaemia (CML) developed three years after the diagnosis of Waldenström macroglobulinemia (WM) in a patient who had received chlorambucil and prednisone during this time. Treatment with busulphan was started and two and a half years later the died in blastic crisis. Six months before the blastic transformation of the leukaemia the amount of IgM, which remains stable (above normal values) along the chronic phase, initiates a decrease, reaching normal values in the phase of acute transformation. CML is a very rare second neoplasia in patients receiving anticancer drugs. The correlation between the decrease of IgM and the blastic crisis is very interesting. We do not find an explanation for this finding although similar observations have been described heralding the appearance of second neoplasias in WM, and one might speculate with a possible role of IgM in the control of chronic phase of the myeloid leukaemia or if the overgrowth of blastic cells could overwhelm the development of lymphoplasmacytoid cells.