['Acute liver failure (ALF) represents a life-threatening situation characterized by sudden and massive liver cell death in the absence of preexisting liver disease. Although most patients require liver transplantation to prevent mortality, some recover spontaneously and show complete liver regeneration. Because of the rarity of this disease, the molecular mechanisms regulating liver regeneration in ALF patients remain largely unknown. In this study, we investigated the role of microRNAs (miRs) that have been implicated in liver injury and regeneration in sera from ALF patients (n\u2009=\u200963). Patients with spontaneous recovery from ALF showed significantly higher serum levels of miR-122, miR-21, and miR-221, compared to nonrecovered patients. In liver biopsies, miR-21 and miR-221 displayed a reciprocal expression pattern and were found at lower levels in the spontaneous survivors, whereas miR-122 was elevated in both serum and liver tissue of those patients. As compared to nonrecovered patients, liver tissue of spontaneous survivors revealed not only increased hepatocyte proliferation, but also a strong down-regulation of miRNA target genes that impair liver regeneration, including heme oxygenase-1, programmed cell death 4, and the cyclin-dependent kinase inhibitors p21, p27, and p57.'] Do microRNAs play a role in spontaneous recovery from acute liver failure?