MicroRNAs (miRNAs) are endogenously expressed RNAs, 18-25 nucleotides in length, that repress protein translation through binding to target mRNAs. miRNAs have been implicated in many cellular processes including cell proliferation, differentiation, and death. Recently, let-7 miRNAs were found to regulate human RAS oncogene expression and to be often down-regulated in human lung tumors. In this study, we examined the expression of let-7 miRNAs in human colon cancer tumors and cell lines, with the result that 2 of 6 cases and 1 of 3 cell lines showed reduced expression of let-7. When let-7 low-expressing DLD-1 human colon cancer cells were transfected with let-7a-1 precursor miRNA, which is located at chromosome 9q22.3, the cells underwent significant growth suppression. At that time, the levels of RAS and c-myc proteins were lowered after the transfection, whereas the levels of both of their mRNAs remained almost unchanged. These findings suggest the involvement of let-7 miRNA in the growth of colon cancer cells. Thus, miRNAs might provide a basis for novel RNA anti-cancer agents.