Dysregulation of microRNA (miRNA) levels is observed in diverse disease states. Earlystudies showed that by analyzing the expression profile of miRNAs in the tissue sampleof a diseased person, it was possible to classify the disease into a specific subtype. Tobe used for diagnostic purposes more practically, however, a less invasive method thantissue biopsy is required. Surprisingly, it was discovered that a notable amount of extracellularmiRNAs circulate throughout the body fluids with high stability. Moreover,the expression profile of miRNAs was shown to differ considerably between healthyand diseased people. In addition, evidence has been accumulating of extracellularmiRNAs acting as signaling molecules between distantly located cells. If the expressionprofile faithfully reflects the disease states, the profiling of extracellular miRNAs willbecome a useful means of early warning or diagnosis of diverse diseases, replacing moreinvasive biopsy methods.