The clinical features of 21 patients (7 men and 14 women, aged 24-65) with human T-lymphotropic virus type I (HTLV-I) associated myelopathy (HAM) in Nagasaki were reviewed. Gait disturbance with spastic paraplegia and bladder dysfunction were the main clinical symptoms. Gait disturbance was observed in all patients and bladder dysfunction was found in 20/21 (95.2%). Mild sensory disturbance was observed in 11/21 (52.3%) patients. Only 4 patients had received blood transfusions before the onset of illness. On the other hand, 6 patients who had family histories of HAM or spastic paraplegia were included as familial clusters of HAM. The study on the T-cell subpopulations revealed a significant increase in the percentage of OKT11+ cells, OKT3+ cells, OKT4+ cells, OKT8+ cells, OKIa1+ cells and IL-2R+ cells. We treated these 21 patients with plasma exchange (PE) and/or prednisolone in an uncontrolled manner. Both treatments were effective (PE: 11/18, prednisolone: 14/20), though the effects were not sufficient. In 5 patients, PE produced rapid and dramatic improvement. Anti-HTLV-I antibodies in serum and cerebrospinal fluid were not correlated with the efficacy of both treatments.