Serum and cerebrospinal fluid (CSF) levels of interferon-gamma (IFN-gamma) were assayed with an immunoenzymetric method in 16 patients with HTLV-I-associated myelopathy (HAM). IFN-gamma levels were elevated in serum of two patients and in CSF of nine. Although there was no significant association between the clinical features and the CSF IFN-gamma levels, the mean levels of CSF IgG and CSF anti-HTLV-I antibody titer were higher in patients with elevated CSF IFN-gamma levels than in those with normal CSF IFN-gamma levels.