In order to clarify the mechanisms of the eosinophilia frequently observed in patients with Hodgkin's disease (HD), 18 patients and 16 age- and sex-matched controls were studied. Increased eosinophil numbers in peripheral blood and serum IgE, as well as decreased cell-mediated immunity were present in HD patients compared with control individuals. Advanced disease was accompanied by lower eosinophil levels, increased IgE, and lower CD4+ T cell counts in peripheral blood. Eosinophilia correlated with CD4+ T cell counts, suggesting that eosinophil production could be under CD4+ T cell control. GM-CSF production in vitro by Phytohemagglutinin-stimulated mononuclear cells was significantly lower in HD patients with eosinophilia. On the other hand, an eosinophil-survival-enhancing activity was found in sera and culture supernatants from controls and HD patients; this activity was stronger for HD patients and was higher for those with eosinophilia. Furthermore, this activity was completely abolished by preincubation with monoclonal antibodies to IL-5, but not with normal mouse serum. Our results suggest that defects of cell-mediated immunity present in patients with HD are accompanied by a predominant type 2 cytokine profile. IL-5 is involved in the increased eosinophil production observed in these patients.