Adult T-cell leukemia is associated with high levels of neopterin, released in large amounts from human macrophages upon stimulation with interferon-gamma. Recent data suggested a potential role of neopterin-derivatives in oxygen radical-mediated processes, and evidence accumulates that oxidative stress is involved in the pathogenesis of viral diseases. We now report that increased concentrations of 7,8-dihydroneopterin may lead to enhanced apoptosis and disturbance of the redox-balance of human leukemic Jurkat T cells. Additionally, we demonstrate that 7,8-dihydroneopterin and hydrogen peroxide activate the type 1 human T-cell leukemia virus (HTLV-1) long terminal repeat (LTR). Furthermore, we found that the activity of the HTLV-1 transactivator protein Tax is amplified by an elevated concentration of 7,8-dihydroneopterin. Tax did not significantly augment 7,8-dihydroneopterin mediated apoptosis. Based on our data we propose that 7,8-dihydroneopterin may be involved in the progression to higher stages of HTLV-1 associated disease.