Thyroid MALT lymphoma is an extremely rare malignancy believed to arise against a background of Hashimoto's thyroiditis. Rituximab is a monoclonal antibody directed against B cell specific antigen CD20. Recently, there have been reports that rituximab is effective for autoimmune thyroid diseases such as Graves' disease as well as for treatment of B cell malignant lymphoma. We present the changes in thyroid autoantibodies in Hashimoto's thyroiditis after rituximab administration for 3 cases of thyroid MALT lymphoma. Case 1 had been taking levothyroxine and was diagnosed with thyroid MALT lymphoma. She was treated with rituximab monotherapy, and her thyroid enlargement improved. Antithyroid peroxidase antibody (TPOAb) turned negative after rituximab monotherapy, and TSH levels decreased with the same levothyroxine dosage. Case 2 was diagnosed with recurrent thyroid MALT lymphoma after chemotherapy (CHOP). He, suffered from leg sensory disturbance because of vincristine sulfate. The patient was treated with rituximab. TPOAb decreased, but did not turn negative. TSH levels were within normal range during the disease course, but TSH levels were low in comparison with before rituximab therapy. Case 3 was diagnosed with thyroid MALT lymphoma after radiation therapy on the neck for laryngeal cancer. Thyroid enlargement improved after rituximab monotherapy, and thyroid autoantibody levels decreased. TSH increased transiently after radiation therapy, but TSH decreased gradually without levothyroxine after rituximab monotherapy. We report 3 cases in which thyroid autoantibody levels in Hashimoto's thyroiditis decreased after rituximab monotherapy for thyroid MALT lymphoma, but it is controversial whether thyroid dysfunction due to Hashimoto's thyroiditis is restored.