The patient with HTLV-I associated myelopathy (HAM) shows a quite uniform clinical picture characterized by slowly progressive spastic paraparesis, slight sensory disturbances and urinary frequency, and the pathogenetic relationship between spastic paraparesis and HTLV-I was established. Since then, the role of the virus in causing myelopathy has drawn increasing attention. However, we have little information about cerebrospinal fluid (CSF) abnormalities in patients with HAM. Analysis of CSF oligoclonal bands (OB) in 22 patients with HAM was reported. All of 22 patients had typical clinical signs and symptoms of HAM with high titers of anti-HTLV-I antibodies in the serum by particle agglutination method. And these antibodies against HTLV-I were confirmed by enzyme-linked immunosorbent assay and western blot. Detection or characterization of CSF OB was done by high resolution agarose gel (HARG) electrophoresis with silver staining and immunofixation method with immunostaining. Other method for detection OB was by agarose isoelectric focusing (IEF), transfer to cellulose nitrate and immunoperoxidase staining (Olsson, 1984). CSF OB was detected in 13 of 22 patients with HAM by the method of immunofixation, using HRAG. All of CSF OB reacted with peroxidase conjugated goat anti-human IgG serum. More than 3 oligoclonal bands were not detected in HRAG electrophoresis. However, CSF OB was detected in all of 22 patients by the method of Olsson (IEF, in agarose, double-antibody peroxidase labelling and avidin-biotin amplification). The majority of patients with HAM had at least 5 or more OB in the region between pH 6.8 and 9.5.(ABSTRACT TRUNCATED AT 250 WORDS)