We evaluated the risk of transformation of asymptomatic immunoglobulin (Ig) M monoclonal gammopathy (aIgM-MG) into symptomatic lymphoproliferative disease in 384 patients, in subgroups of patients with IgM MG of undetermined significance (MGUS) and smoldering Waldenstrom's macroglobulinemia (sWM). One hundred seventy-two patients with aIgM-MG with bone marrow (BM) histopathology and immunophenotyping were available for analysis. After a median of 45 months (range, 12-233 months), 45 cases of aIgM-MG (11.7%) evolved into lymphoproliferative disease, as follows: symptomatic WM (n = 41), non-Hodgkin's lymphoma (NHL; n = 2), IgM multiple myeloma (IgM-MM; n = 1), and primary amyloidosis (n = 1). Cumulative transformation probability at 5 and 10 years was 8% and 29%, respectively. The parameters significantly correlated with evolution were, at univariate analysis, BM lymphoplasmacytic infiltration, high erythrocyte sedimentation rate, hemoglobin (Hb) level, IgM size, and peripheral lymphocytosis; at multivariate analysis, the parameters were IgM size and peripheral lymphocytosis, with Hb level associated with a trend toward higher progression risk. Of the 138 cases of IgM-MGUS, 14 (10.1%) evolved (13 WM, 1 IgM-MM) after a median of 75 months (range, 12-117 months); of the 34 cases of sWM, 13 (38.2%) progressed to WM after 55 months (range, 13-154 months). In patients with IgM-MGUS, event-free survival at 5 and 10 years was 95% and 83%, respectively, and in patients with sWM, 77% and 42%, respectively (P = 0.0001). Among patients with aIgM-MG, those at high risk of evolution are patients with sWM, a distinct entity with clear BM evidence of NHL.