While global microRNA（miRNA）expression patterns of many embryologic, physiologic, and oncogenic processes have been described, description of the role of miRNAs for preoperative chemoradiotherapy（CRT）in rectal cancer is lacking. Our purpose of this study was to define the expression pattern of miRNAs for prediction of response to chemoradiotherapy in rectal cancer. Rectal cancer patients（n＝22）who underwent preoperative CRT（40Gy radiotherapy combined with S-1）were studied. S-1 is a novel oral fluoropyrimidine inhibitory for dihydropyrimidine dehydrogenase and has potent radiosensitizing property. RNA harvested from biopsy specimens of rectal cancer before preoperative CRT was hybridized to miRNA microarrays（８２１genes）. Response to CRT was determined by histopathologic examination（Japanese Society for Cancer of the Colon and Rectum）of surgically resected specimens and RECIST. Groups were classified as responders（ grade 2 or 3, CR or PR）or nonresponders（grade 0 or 1, SD or PD）, respectively. Response to CRT determined by histopathologic examination of surgically resected specimens and RECIST were as follows : responders（grade 2 or 3, n＝15）,（PR, n＝１４）, nonresponders（grade 0 or 1, n＝7）（, SD, n＝8）. Response rate was６８％（grade 2 or 3）and 63％（PR）. Two miRNAs（miR- 142, 223）with increased expression were identified that correctly differentiated responders from nonresponders of CRT by histopathologic examination. One overexpressed（miR-223）and 4 underexpressed miRNAs（miR-17, 20, 92, 106）differentiated responders from nonresponders of CRT by RECIST. Rectal cancer may have a distinct miRNA expression to predict pathological response to preoperative chemoradiotherapy
