Parasite-specific humoral and cellular immune responses were evaluated in in nine immunosuppressed patients with Strongyloides stercoralis infection. Four patients had the disseminated form of the disease, and five had serious manifestations without evidence of hyperinfection. All patients had high levels of immunoglobulin G (IgG) antibodies directed against S. stercoralis larval antigens as detected by the enzyme-linked immunosorbent assay. Seven patients had both elevated total serum IgE and parasite-specific IgE antibodies detected by a modified radioimmunoassay. The hyperinfected patients differed significantly from the nonhyperinfected group in total and specific IgE levels and peripheral eosinophilia but not in specific IgG levels. None of the patients tested exhibited significant in vitro lymphoproliferative responses to parasite antigens. Our results indicated that the possibility of a protective role of IgE and eosinophils in strongyloidiasis deserves further investigation. In addition, we suggest that the enzyme-linked immunosorbent assay for specific IgG antibodies may have an important place in the diagnosis of strongyloidiasis in immunocompetent as well as in immunocompromised patients.