MGUS is found in more than 1 % of people 50 years or older and in approximately 3% of those older than 70 years. Of these the great majority are of IgG or IgA type while IgM MGUS are less frequent.The aim of this study was to analyse the natural history of IgM MGUS and the risk of progression to lymphoproliferative disorders. From July 1973 to June 2004, we observed 323 monoclonal gammopathies of IgM type. At diagnosis, 198 (61%) were MGUS, 111 (34%) were Waldeström’s Macroglobulinemia (WM) of whom 11 were pretreated, 8 (2,4%) were non Hodgkin Lymphoma (NHL), 5 (1,5%) were IgM Myeloma and only 1 was a chronic lymphocytic leukaemia. Of 198 patients with IgM MGUS, 125 (63%) were men and 73 (37%) were women ( ratio 1.71).Median age at diagnosis was 65.7 (range 33–89 years). At diagnosis, all patients had an IgM concentration less than 2 gr/dl with a median of 1.7 g/dl. Median Hb, serum creatinine and albumin levels were: 13.2 gr/dl, (range 6.9–17.9), 1,04 mg/dl ( range 0.6– 2,5) and 3.9 gr/dl ( range 2.4–5). Hepatomegaly as well as splenomegaly were present in 24% and 14% of patients, respectively. The 198 patients were monitored for 8741 months/person (median 25 months, range 0–411). During The follow up a lymphoproliferative disorder developed in 17 (8.5%) patients. In particular 11 pts (5.5%) pts evolved to WM and 6 (3%) to NHL. Progression incidence to linfoproliferative disorders was 0.001 months/person. Cumulative incidence of progression to lymphoproliferative disease was 1.3 % at 1 years, 6.8 % at 5 years, 23.2% at 10 years and 45.4% at 15 years. In conclusion the patients with IgM MGUS are more likely to die of unrelated diseases than to have progression to a malignant lymphoproliferative disorder.