BACKGROUND AND OBJECTIVES: Since the first description of pathology of the kidney in Waldenstr?isease in 1970, there have been few reports on kidney complications of IgM-secreting monoclonal proliferations. Here, we aimed to revisit the spectrum of renal lesions occurring in patients with a serum monoclonal IgM. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Fourteen patients with a circulating monoclonal IgM and a kidney disease related to B cell proliferation were identified retrospectively. Demographic, clinical, and laboratory data were assessed for each patient at the time of kidney biopsy. RESULTS: Seven patients had a nephrotic syndrome. Patients without nephrotic syndrome all had impaired renal function. Mean serum creatinine was 238 micromol/L. For five patients, the diagnosis of monoclonal IgM preceded the kidney disease by 28.8 mo (range 12 to 60). Seven patients had Waldenstr?isease, two had a small B cell non-Hodgkin lymphoma, one had an IgM-excreting multiple myeloma, one had a marginal zone B cell lymphoma, and three had an IgM-related disorder. Renal lesions included (1) intracapillary monoclonal deposits disease with granular, electron-dense IgM thrombi occluding capillary lumens (5); (2) atypical membranoproliferative glomerulonephritis (3); (3) lambda light chain amyloidosis (2) associated with mu deposits in one patient; (4) acute tubular necrosis (1); and (5) CD20(+) lymphomatous infiltration (3). Remission of the nephrotic syndrome was attained in three of seven patients, and renal function improved after chemotherapy. CONCLUSIONS: Although renal complications of IgM proliferations are rare, a wide spectrum of kidney lesions is observed, without correlation with the type of hematologic disorder.