Human T lymphotropic virus-type 1 (HTLV-1) is the causal agent of the HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), adult T cell leukaemia/lymphoma and infective dermatitis associated with HTLV-1 (IDH). Over-production of proinflammatory cytokines and an increase in HTLV-1 proviral load are features of HAM/TSP, but the immunological basis of IDH has not been established. In addition to severe cutaneous manifestations, the importance of IDH relies on the observation that up to 30% of children with IDH develop HAM/TSP in childhood and adolescence. In this study we determined the immune response in patients with IDH measuring interleukin (IL)-4, IL-5, IL-10, interferon (IFN)-γ and tumour necrosis factor (TNF)-α levels as well as the HTLV-1 proviral load. Additionally, regulatory cytokines and anti-cytokines were added to cultures to evaluate the ability of these molecules to down-modulate TNF-α and IFN-γ synthesis. HTLV-1 carriers and patients with HAM/TSP served as controls. TNF-α and IFN-γ levels were higher in IDH than in HTLV-1 carriers. There was no difference in IFN-γ and TNF-α concentrations in IDH and HAM/TSP patients. There was a tendency for higher IL-4 mRNA expression and immunoglobulin E (IgE) levels in IDH than in HTLV-1 carriers, but the difference did not reach statistical significance. The HTLV-1 proviral load was significantly higher in IDH patients than in HTLV-1 carriers. IDH is characterized by an exaggerated Th1 immune response and high HTLV-1 proviral load. The similarities between the immunological response in patients with IDH and HAM/TSP and the high proviral load observed in IDH provide support that IDH is a risk factor for development of HAM/TSP.