A 55-year-old female of Cushing's disease associated with multiple myeloma is reported. The association of the two diseases has not been previously noted. Elevated cortisol level was controlled successfully by low doses of adrenocorticolytic agent, o,p'-DDD. However, the exacerbation of multiple myeloma was found to accompany the serum cortisol normalization. Previous reports have shown the good efficacy of combination therapy with melphalan and prednisolone for multiple myeloma. It was suggested that the decrease of endogenous cortisol level might exacerbate the patient's multiple myeloma.