We report a case of a 47-year-old man with multicentric Castleman's disease (MCD) and progressive renal dysfunction due to mesangial proliferative glomerulonephritis, possibly from IgA nephropathy. At age 36 years, he was referred to a hematologist due to hypergammaglobulinemia. Because of systemic lymph node swelling, he underwent right cervical lymph node biopsy at age 41 years and MCD (plasma cell type)was diagnosed. During this period, microscopic hematuria and persistent proteinuria occurred and his renal function deteriorated (serum creatinine (Cr) rising from 0.7 mg/dL to 1.4 mg/dL). Treatment with intravenous methylprednisolone at the dose of 1 g daily for 3 days followed by oral prednisolone at 20 mg daily reduced his lymphadenopathy and improved the renal function. However, his renal function deteriorated again, from Cr 0.8 mg/dL to 1.8 mg/dL over 6 years in line with gradual prednisolone tapering to 6 mg daily. At age 47 years, he was referred to our nephrology department and underwent a renal biopsy. The microscopic examination showed IgA nephropathy with crescent formation, accompanied by mild lymphoplasmacytic tubulointerstitial nephritis. Treatment with the same dose of intravenous methylprednisolone therapy followed by oral prednisolone at 40 mg daily, improved his proteinuria, hematuria and renal dysfunction. The coexistence of MCD and IgA nephropathy is a rare phenomenon. In addition, IL-6, overproduced by MCD might have influenced the mesangial cell proliferation and the activity of IgA nephropathy in the present case.