In a recent study, Brennecke and colleagues show that bantam, a previously elusive Drosophila gene that promotes tissue growth, encodes a microRNA. The bantam microRNA regulates cell number in vivo and inhibits cell death by binding to the mRNA of the pro-apoptotic gene hid. These findings demonstrate that a single, tiny Drosophila RNA can regulate organ and organism size and link the nascent field of microRNA function to the study of pathways that coordinate cell death, cell growth and cell proliferation during the development of metazoan organisms.