A postmarketing surveillance study was undertaken to investigate the safety and efficacy of interferon-alpha for human T-cell lymphotropic virus type 1 (HTLV-1)-associated myelopathy (HAM) under routine treatment conditions. A total of 273 cases from 91 medical institutions were registered into the survey. So far, 167 cases had been evaluated for safety and 152 for efficacy. The efficacy evaluation was rated based on clinical symptoms of HAM. Efficacy ratio (rate of patients assessed as "modest to markedly improved" and "mildly improved") at 4 weeks was 66.2%. Factors that significantly affected efficacy ratio at 4 weeks was initial Osame's motor disability score (OMDS) before interferon-alpha therapy and duration and stage of illness. Sustained improvement of OMDS for at least 5 months after stopping interferon-alpha was observed in 11 of 30 patients (36.7%). A total of 536 adverse drug reactions (ADRs) occurred in 146 patients, 46 of which were serious. Because some of these ADRs occurred late, it is necessary to watch out for them during long-term treatment.