The detection of circulating tumor cells (CTCs) has received great attention. 
MicroRNA-21 (miR-21) plays crucial roles in carcinogenesis and is considered as 
one of the most studied oncomiRNAs. We determined if miR-21 could be used a 
marker for the detection of CTCs in gastric cancer patients. Peripheral blood 
samples were collected from 53 preoperative patients with gastric cancer and 20 
healthy volunteers. Real-time reverse transcription-polymerase chain reaction 
was used to detect the level of miR-21. Receiver operator characteristic curves 
(ROC) were constructed. Patients with gastric cancer display a significantly 
higher level of miR-21 in peripheral blood than those from controls. The miR-21 
level was associated with the tumor node metastasis (TNM) stage, tumor size and 
tissue categories. The area under ROC curve was up to 0.853 ± 0.086. This study 
highlights the potential of the detection of miR-21 in peripheral blood as a 
novel tool for monitoring CTCs in gastric cancer patients.