A 45-year-old female was admitted to our hospital complaining of abnormal sensation in the upper and lower extremities. She was given a diagnosis of smouldering type adult T cell leukemia 9 months before admission. On admission, neurological examination revealed sensory disturbance of polyneuropathy type, and generalized hyporeflexia. Nerve conduction study demonstrated decreased F wave appearance rate. Motor unit potentials in electromyelography had long duration and high amplitude. MR imaging of the brain showed multiple abnormal lesions in the periventricular and subcortical white matter. CSF protein was mildly increased, and anti-HTLV-I antibody was positive. Treatment with corticosteroid was effective, resulting in disappearance of sensory symptoms and normalization of F wave appearance. The present case is different from HAM, and the above clinical features may fit to the disease entity of HTLV-I associated polyradiculoneuropathy and cerebral white matter lesions.