We have successfully treated multiple myeloma of IgD (lambda) type [IgD (lambda) -MM] by natural alpha-interferon (alpha-IFN) single therapy. A 45 year-old man was admitted to Tokyo Medical College Hospital because of general fatigue in August, 1989. Immunoelectrophoresis, bone marrow biopsy and systemic bone survey revealed IgD (lambda) -MM with Bence Jones (BJ) proteinuria and slightly osteolytic lesions. We started treating him with natural alpha-IFN single therapy. Three months later, serum IgD markedly decreased and BJ proteinuria disappeared. Bone marrow, which had been packed with myeloma cells at the admission, was almost replaced by normal hematopoietic cells. In April 1990, he is still free of disease with only alpha-IFN single therapy. This result might suggest that alpha-IFN single therapy is effective for IgD-MM.