We report the immunological studies of 50 cases with a chronic progressive myelopathy associated with elevated antibodies to human T-cell lymphotropic virus type I (HTLV-I) and adult T-cell leukemia-like cells (HAM). These are as follows: (1) T-cell activation; (2) increase of the OKT4/OKT8 ratio, due mostly to increased inducer/helper T cells and/or decreased suppressor/cytotoxic T cells; (3) decrease of natural killer cell numbers and activity; and (4) increase of immunoglobulins (IgG, IgA). Our results provide evidence that autoimmune events may participate in the pathogenesis of HAM and may be relevant in the hypothesis of common immune mechanisms between HAM and HTLV-I positive tropical spastic paraparesis.