BACKGROUND:Kidney diseases associated with immunoglobulin M (IgM) monoclonal gammopathy are poorly described, with few data for patient outcomes and renal response.STUDY DESIGN:Case series.SETTING & PARTICIPANTS:35 patients from 8 French departments of nephrology were retrospectively studied. Inclusion criteria were: (1) detectable serum monoclonal IgM, (2) estimated glomerular filtration rate (eGFR) 0.5g/d and/or microscopic hematuria, and (3) kidney biopsy showing monoclonal immunoglobulin deposits and/or lymphomatous B-cell renal infiltration. All patients received chemotherapy, including rituximab-based regimens in 8 cases.PREDICTORS:Patients were classified into 3 groups according to renal pathology: glomerular AL amyloidosis (group 1; n=11), nonamyloid glomerulopathies (group 2; n=15, including 9 patients with membranoproliferative glomerulonephritis), and tubulointerstitial nephropathies (group 3; n=9, including cast nephropathy in 5, light-chain Fanconi syndrome in 3, and isolated tumor infiltration in 1).OUTCOMES:Posttreatment hematologic response (≥50% reduction in serum monoclonal IgM and/or free light chain level) and renal response (≥50% reduction in 24-hour proteinuria or eGFR≥30mL/min/1.73m2 in patients with glomerular and tubulointerstitial disorders, respectively).RESULTS:Nephrotic syndrome was observed in 11 and 6 patients in groups 1 and 2, respectively. Patients in group 3 presented with acute kidney injury (n=7) and/or proximal tubular dysfunction (n=3). Waldenström macroglobulinemia was present in 26 patients (8, 12, and 6 in groups 1, 2, and 3, respectively). Significant lymphomatous interstitial infiltration was observed in 18 patients (4, 9, and 5 patients, respectively). Only 9 of 29 evaluable patients had systemic signs of symptomatic hematologic disease (2, 5, and 2, respectively). In groups 1, 2, and 3, respectively, hematologic response was achieved after first-line treatment in 3 of 9, 9 of 10, and 5 of 6 evaluable patients, while renal response occurred in 5 of 10, 9 of 15, and 5 of 8 evaluable patients.LIMITATIONS:Retrospective study; insufficient population to establish the impact of chemotherapy.CONCLUSIONS:IgM monoclonal gammopathy is associated with a wide spectrum of renal manifestations, with an under-recognized frequency of tubulointerstitial disorders.