Abstract: Micro-ribonucleic acids (miRNAs) are endogenous single-stranded small non-coding RNAs. miRNAs bind to a com-plementary site in the 3' untranslated region of their target mRNAs through canonical base pairing, which can direct the degradation or translational repression of these transcripts. Thus, miRNAs can effectively silence the protein expression of target genes post-transcrip -tionally. miRNAs may also regulate the expression of oncogenes and tumor suppressor genes and could be involved in almost all known hallmarks of cancinogenesis. In this paper, we discuss the following in detail: ( 1) biogenesis and main functions of cellular miR-NAs, (2) stability and detectability of exosomal miRNAs in biological fluids; and ( 3) feasibility of miRNAs as a potential new class of biomarkers derived from urinary exosome in the malignancy of urinary system. Finally, we summarize studies on urinary exosomal miRNAs as potential biomarkers of prostate, bladder, and kidney cancers.