A 64 year-old man began to feel numbness on his bilateral feet in 1990. He was diagnosed as diabetes mellitus with a high fasting glucose level of 580 mg/dl in 1993 and he received oral hypoglycemic agents. Since then, his blood glucose levels had been in good control under diet therapy and medication. However, his numbness worsened and progressive weakness of bilateral lower legs occurred in 1997. Bilateral anterior tibial muscles were atrophic and deep tendon reflexes were decreased on bilateral upper and lower limbs. Protein level of his cerebrospinal fluid was 63 mg/dl. Nerve conduction study fulfilled the electrophysiological diagnostic criteria of CIDP. Superficial peroneal nerve biopsy showed loss of myelinated fibers, small amount of onion bulbs and thickening of the basement membrane of arterioles. Demyelination was predominant in teased fiber study. These findings were compatible with CIDP combined with diabetes mellitus (DM-CIDP). His numbness and leg weakness improved after intravenous high dose immune globulin therapy. DM-CIDP must be distinguished from diabetic peripheral polyneuropathy because immunological therapy may be effective in DM-CIDP patients.