The efficacy of long-term, high dose interferon-alpha (IFN-alpha) therapy was studied in seven patients with HTLV-I-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). IFN-alpha was administered at a dose of 6x10(6) international units daily for the initial 2 weeks and thereafter 3 times a week for the following 22 weeks. Five patients showed a sustained improvement in motor performance during and up to 6 months after the completion of IFN-alpha. The other patient who responded to IFN-alpha initially dropped out at 3 months because of depression, while another patient first deteriorated and thereafter dropped out. In the six responders, the absolute number of peripheral blood lymphocytes (PBL) harboring the HTLV-I genome as evaluated by the quantitative polymerase chain reaction method decreased significantly during the therapy period (28.6+/-16.6% reduction, P=0.0083), whereas the one deteriorated patient showed a 2.5-fold increase in HTLV-I-infected cells. The autoproliferation of CD4(+) T clone cells from a single cell culture was markedly depressed even after the cessation of IFN-alpha in the responders who completed long-term IFN-alpha therapy. In addition, the CD8(+)DR(+) T cells in the peripheral blood and soluble IL-2 receptor levels in the sera increased significantly during the therapy in all patients (P=0.0431 and P=0.0041, respectively). Therefore, the results of our study suggested that both the reduction of HTLV-I proviral DNA load and immunomodulation by long-term IFN-alpha therapy contributed to its sustained clinical benefits. (C) 1997 Elsevier Science B.V.