Hashimoto's thyroiditis is associated with myeloproliferative and lymphoproliferative neoplasms. The risk of carcinoma of the thyroid gland is increased in these patients. Furthermore, multiple myeloma can present together with some autoimmune diseases. We report the case of a 57-year-old woman with Hashimoto's thyroiditis who developed multiple myeloma with myeloma nephropathy. Her renal function deteriorated to end stage and she required maintenance hemodialysis. Although autoimmune disorder might play an important role in lymphomagenesis in patients with Hashimoto's thyroiditis, it is not known whether the chronic inflammation that takes place in Hashimoto's thyroiditis stimulates the development of multiple myeloma. The pathogenetic mechanisms responsible for the development of multiple myeloma in patients with Hashimoto's thyroiditis remain unclear.