Background: Human T cell lymphotropic virus type 1 (HTLV-1) infection has been associated with recurrent and disseminated strongyloidiasis and adult T cell leukemia/lymphoma (ATLL). Methods:We compared immunological aspects and markers for ATLL in HTLV-1 patients with or without strongyloidiasis, and evaluated the influence of Strongyloides stercoralis treatment on the immune response and clinical outcomes of HTLV-1 infection.Results: Levels of TNFa and IFNg were lower in patients coinfected with HTLV-1 and S. stercoralis than in patients with HTLV-1 only (p , 0.05), and there was an increase in TNFa levels after anthelmintic treatment. Levels of sIL-2R were higher in patients with HTLV-1 coinfected with S. stercoralis and anthelmintic treatment decreased sIL-2R levels (p , 0.05). The one patient who developed ATLL was coinfected with S. stercoralis. Conclusion:These data show that helminthic infection has a modulatory role in HTLV-1 infection and that S. stercoralis may be a cofactor in the development of ATLL.