Cystoisospora belli, formerly Isospora belli, as an opportunistic infection agent, is seen in immunocompromised patients like HTLV-1. We describe here cystoisosporiasis in an HTLV1 Iranian female in Mashhad, northwestern Iran in 2012 who presented with a debilitating diarrheal illness and great weight loss. C. belli was detected in her stool by modified acid-fast staining and then by molecular detection. Serologic testing was negative for HIV but she showed positivity for HTLV-1 infection. Treatment with TMP/SMX led to improvement of her diarrhea but she died after one year due to malabsorption syndrome. Adequate detection of C. belli diarrhea in immunocompromise patients of HTLV1 in endemic area can be cured by TMP/SMX.