The significance of IgM on immunofluorescence in renal biopsy specimens remains unclear. This retrospective case study was conducted to define the clinical features, response to therapy and outcome of patients with Mesangioproliferative Glomerulonephritis (MGN) with diffuse IgM deposition. Of 1919 native renal biopsies performed over a ten-year period, 139 (7.2%) had light microscopic features of MGN and manifested IgM as the dominant immunoglobulin. When exclusion criteria (more than a trace of IgA or IgG, segmental IgM, evidence of SLE, vasculitis, FSGS or Alport's syndrome and pregnant patients) were applied, 60 patients (3.1%) remained. Follow-up data were available for 54 cases with a mean age of 26.5 years (range 1.7-63). Mean follow-up period was 7.4 years (range 4.7-22.2). Forty-one per cent presented with nephrotic syndrome (NS), 26% with asymptomatic proteinuria (>250mg/24hr), 18% with macroscopic hematuria and 15% with isolated microscopic hematuria. Twenty-one percent of patients were hypertensive at presentation. Creatinine was initially <120 (mol/L in all but one patient. Only four patients (7.4%), all nephrotic, suffered a decline in renal function despite treatment; all 4 developed ESRF after a mean of 5.6 years (range 2-8.3). Two of these were subsequently re-biopsied and found to have FSGS. No patients with isolated microscopic / macroscopic hematuria or asymptomatic proteinuria suffered a decline in renal function. Protein excretion rate fell into the normal range in 63% of those receiving steroids, with 82% becoming steroid dependent. Of those treated with cyclosporine (48%) or cyclophosphamide (52%) only 9.5% and 14.5% respectively remained in prolonged remission after discontinuing treatment. It is concluded that MGN with IgM deposition carries a very favorable prognosis except in patients with NS who develop FSGS. However there is a high incidence of steroid dependence and resistance in the proteinuric group.