Human T-cell leukemia virus type 1 (HTLV-1) is associated with adult T-cell leukemia-lymphoma (ATLL) in about 5% of infected individuals. Coinfection by Strongyloides stercoralis has been suggested to be a cofactor for development of ATLL. We describe a patient who presented with HTLV-1-associated chronic ATLL and Strongyloides infection. Studies of this patient's viral RNA levels demonstrated stimulation of HTLV-1 replication by Strongyloides, which resolved with anti-helminthic therapy. This case provides support for the hypothesis that Strongyloides is a cofactor for ATLL via T-cell stimulation.