Hyperinfection with Strongyloides stercoralis occurred in three patients with malignant lymphoma. The probable cause in each case was alteration of the immune responses either as a result of the malignant lymphoma or by the treatment given. Though thiabendazole (Mintezol) has produced reasonable results in the treatment of the hyperinfection syndrome, preliminary data suggest that the new broad-spectrum anthelmintic levamisole (Ketrax) is more effective. All patients who live or have lived in an area where strongyloidiasis is endemic should be investigated to detect the presence of the nematode before and during treatment with drugs with immunosuppressive properties. In view of the high mortality with S. stercoralis hyperinfection, vigorous therapy should be instituted before the use of immunosuppressive drugs.