Waldenstrom's macroglobulinemia (WM) is a poorly characterized B-cell lymphoproliferative disorder. There are a relatively limited number of detailed clinicopathological assessments, while the majority of clinical trials have been nonrandomized, single-institution phase II studies. Unfortunately progress in this disorder has been hindered by a lack of universally accepted diagnostic criteria. It is clear that criteria incorporating clinical, morphological, immunophenotypic, and, ultimately, genotypic parameters are needed for future clinical trails. Following a detailed clinicopathological assessment of 111 patients and a review of the published literature the following diagnostic criteria were proposed for WM: IgM monoclonal gammopathy of any concentration, bone marrow infiltration by small lymphocytes, plasmacytoid cells and plasma cells in a diffuse, interstitial or nodular pattern, and a surface Ig(+)CD19(+)CD20(+)CD5(-)CD10(-)CD23(-) immunophenotype.