Objective: Downregulation of specific microRNAs (miRNAs) occurs in human tumors, which suggests a function for miRNAs in tumor suppression. We investigated the role of the miRNAs miR-143 and miR-145 in gastric cancers. Methods: The expression levels of miR-143 and miR-145 in the samples from 43 patients with gastric cancer were determined by real-time PCR using TaqMan assay. The growth inhibitory effect was estimated by the transfection of human gastric cancer cells with the miRNA. Results: The expression levels of miR-143 and -145 were decreased in most human gastric cancers examined, as previously reported to occur in colon tumors. The transfection of human gastric MKN-1 cells with miR-145 resulted in a greater growth inhibitory effect than that with miR-143, results which were contrary to those in colon cancers. In MKN-1 cells, an additive effect on growth inhibition was shown by the combined transfection with miR-143 and miR-145; further, higher sensitivity to 5-fluorouracil was also observed following the transfection with miR-143 or miR-145. The possible candidate target messenger RNAs of miR-145 were identified to be insulin receptor substrate-1 and β-actin. Conclusion: Taken together, these findings suggest that miR-143 and miR-145 act as anti-oncomirs common to gastrointestinal tumors.