The present study was conducted to determine whether interferon (IFN)-gamma production by CD4(+) cells in patients infected with human T cell lymphotropic virus (HTLV) is associated with expression of Tax, an HTLV type 1 (HTLV-1) transactivator. The frequency of IFN-gamma production from CD4(+) cells was greater in HTLV-1-infected patients (n=21) than in uninfected (n=3) and Strongyloides stercoralis-infected patients (n=4), and greater in patients with HTLV-1 with detectable Tax than in patients with HTLV-1 with undetectable Tax. In the patients with HTLV-1 with detectable Tax, the majority of CD4(+) cells making IFN-gamma did not express Tax.