['CD133 is a marker that identifies/enriches cancer stem cell implicated in tumor initiation. We hypothesize that changes in the CD133 mRNA expression levels and vascular endothelial growth factor (VEGF) may correlate tumor response in GIST.', 'After informed consent, we obtained peripheral blood samples from 24 evaluable patients with gastrointestinal stromal tumors (GIST). There were 7 -paired samples before and after treatment, We measured CD133 mRNA levels by real time RT-PCR method and vascular endothelial growth factor (VEGF) levels by ELISA. All measurements were done in duplicates in two separate experiments. The treatment resulted in significant reduction of CD133 mRNA expression (p\u200a=\u200a0.048) as well as the level of VEGF (p\u200a=\u200a0.003). The mean CD133 mRNA levels for GIST patients was 615. We found no correlation between the CD133 mRNA levels and VEGF levels. (p\u200a=\u200a0.826). Logistic regression analysis suggested a relationship between elevated CD133 mRNA levels and fitted probability of eventual progressive disease (PD) and mixed response at 37% for CD133 mRNA of 2.25, and the probability of eventual PD/MR is 84% for a CD133 of 2072 (p\u200a=\u200a0.08).'] Does peripheral blood mononuclear CD133 mRNA levels correlate with response to treatment in patients with gastrointestinal stromal tumors?