Background: The association of severe strongyloides with HTLV-I is well known; however, the seroprevalence of HTLV-I in other groups with strongyloidiasis is still unknown. We conducted a prospective study in patients with intestinal strongyloidiasis without known immunodepression who failed to respond to standard therapy with ivermectin or thiabendazole (failure was defined as one positive stool examination at the post-therapy follow up). All these patients were tested for HTLV-I by ELISA and Western Blot. Results: Forty seven patients were evaluated: 74.5% (35 out of 47) were HTLV-I positive, without significant difference between males (76%) and females (72.7%). Conclusions: We recommend that all patients with uncomplicated intestinal strongyloidiasis, who fail standard therapy, be studied for HTLV-I infection.