Tehran University of Medical Sciences Iranian Journal of Parasitology 1735-7020 11 1 2016 03 14 121 125 Reza SHAFIEI Vector-borne Diseases Research Center, North Khorasan University of Medical Sciences, Bojnurd, Iran AND Dept. of Parasitology and Mycology, School of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran Mohsen NAJJARI Dept. of Parasitology and Mycology, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran Ali KARGAR KHEIRABAD Dept. of Virology, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran Golamreza HATAM Basic Sciences in Infectious Diseases Research Center, Shiraz University of Medical Sciences. Shiraz, Iran 2016 03 14 2016 03 14 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. https://ijpa.tums.ac.ir/index.php/ijpa/article/view/820 https://ijpa.tums.ac.ir/index.php/ijpa/article/download/820/664