Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Frontier Medicine. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Neurology. Kyoto Prefectural University of Medicine Graduate School of Medical Science. Kyoto. Department of Neurology and Geriatrics. Kagoshima University Graduate School of Medical and Dental Sciences. Kagoshima, Japan. Department of Neurology. Fukuoka University. Fukuoka, Japan. Department of Neurology. Fukuoka University. Fukuoka, Japan. Department of Cardiovascular Medicine. Nephrology and Neurology. Graduate School of Medicine. University of the Ryukyus, Okinawa, Japan. Department of Cardiovascular Medicine. Nephrology and Neurology. Graduate School of Medicine. University of the Ryukyus, Okinawa, Japan. Department of Pharmacology. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Pharmacology. St. Marianna University School of Medicine. Kawasaki, Japan. Translational Research Center for Medical Innovation, Foundation for Biomedical Research and Innovation at Kobe. Kobe, Japan. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki. Japan. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki. Japan. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan. Department of Advanced Medical Innovation. St. Marianna University Graduate School of Medicine. Kawasaki, Japan / Division of Hematology and Oncology. Department of Medicine. University of California Irvine School of Medicine. Irvine, CA, USA. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Department of Internal Medicine. Division of Bone and Mineral Diseases. Washington University School of Medicine. St. Louis, MO, USA. Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. SHIN-YURIGAOKA General Hospital. Kanagawa, Japan. Fundação Oswaldo Cruz. Instituto Nacional de Infectologia Evandro Chagas. Laboratório de Pesquisa Clínica em Neuroinfecções. Rio de Janeiro, RJ, Brasil. Viral immunology Section. Neuroimmunology Branch. National Institute of Neurological Disorders and Stroke. National Institutes of Health. Bethesda, MD, USA. Escola Bahiana de Medicina e Saúde Pública. Salvador, BA, Brasil / Fundação Oswaldo Cruz. Instituto Gonçalo Moniz. Salvador, BA, Brasil. Escola Bahiana de Medicina e Saúde Pública. Salvador, BA, Brasil / Fundação Oswaldo Cruz. Instituto Gonçalo Moniz. Salvador, BA, Brasil. Department of Health Sciences. Seebohm Rowntree Building. University of York. York , UK. Section of Virology. Department of Infectious Disease. Imperial College London. London, UK. School of Public Health. Faculty of Medicine. University of Queensland. 266 Herston Road. Herston, Australia. Department of Rare Diseases Research. Institute of Medical Science. St. Marianna University School of Medicine. Kawasaki, Japan / Division of Neurology. Department of Internal Medicine. St. Marianna University School of Medicine. Kawasaki, Japan.Practical Research Project for Rare/Intractable Diseases of the Japan Agency for Medical Research and Development. Ministry of Health, Labour and Welfare of Japan.Corticosteroids are most commonly used to treat HTLV-1-associated myelopathy (HAM); however, their clinical efficacy has not been tested in randomized clinical trials. This randomized controlled trial included 8 and 30 HAM patients with rapidly and slowly progressing walking disabilities, respectively. Rapid progressors were assigned (1:1) to receive or not receive a 3-day course of intravenous methylprednisolone in addition to oral prednisolone therapy. Meanwhile, slow progressors were assigned (1:1) to receive oral prednisolone or placebo. The primary outcomes were a composite of ≥1-grade improvement in the Osame Motor Disability Score or ≥30% improvement in the 10 m walking time (10 mWT) at week 2 for rapid progressors and changes from baseline in 10 mWT at week 24 for slow progressors. In the rapid progressor trial, all four patients with but only one of four without intravenous methylprednisolone achieved the primary outcome (p = 0.14). In the slow progressor trial, the median changes in 10 mWT were −13.8% (95% CI: −20.1–−7.1; p < 0.001) and −6.0% (95% CI: −12.8–1.3; p = 0.10) with prednisolone and placebo, respectively (p for between-group difference = 0.12). Whereas statistical significance was not reached for the primary endpoints, the overall data indicated the benefit of corticosteroid therapy.