Taking into consideration some statements about better efficacy and good tolerability of mebendazole and since thiabendazole has not been produced in our country the past few years we have conducted a study evaluating mebendazole, in comparison with thiabendazole in the treatment of patients with strongyloidiasis. Strongyloidiasis is a disease that should be treated with an effective and active drug since it can rapidly progress and be fatal in patients with disturbed immunocompetence. One hundred and ten patients with strongyloidiasis were treated with oral thiabendazole in a dosage of 50 mg/kg daily for two days; the other group of 41 patients was given mebendazole in a dosage of 10 mg/kg/day orally for five days. Clinical evaluations, parasitologic and hematologic tests were performed within three months after the therapy. Patients were considered to have been cured if parasitologic findings were negative and abnormal blood eosinophilia decreased below 0.09 (733/microliters). According to these criteria thiabendazole was effective in 96.4% of patients and mebendazole in 44% of patients only. We conclude that thiabendazole has still to be regarded the drug of choice in treating patients with strongyloidiasis. Mebendazole is far less effective in patients with this helminthiasis and very probably only suppresses the infection. The reports of other studies on the effect of some of the newer benzimidazole antihelmintics as cambendazole, albendazole and flubendazole have shown that they are toxic or less effective in the treatment of strongyloidiasis.