Recently, genome-wide association studies have implicated the human LIN28B locus in regulating height and the timing of menarche^,,,,. LIN28B and its homolog LIN28A are functionally redundant RNA-binding proteins that block biogenesis of let-7 microRNAs^,,,. lin-28 and let-7 were discovered in Caenorhabditis elegans as heterochronic regulators of larval and vulval development but have recently been implicated in cancer, stem cell aging and pluripotency^,,,. The let-7 targets Myc, Kras, Igf2bp1 and Hmga2 are known regulators of mammalian body size and metabolism^,,,,. To explore the function of the Lin28–Let-7 pathway in vivo, we engineered transgenic mice to express Lin28a and observed in them increased body size, crown-rump length and delayed onset of puberty. Investigation of metabolic and endocrine mechanisms of overgrowth in these transgenic mice revealed increased glucose metabolism and insulin sensitivity. Here we report a mouse that models the human phenotypes associated with genetic variation in the Lin28–Let-7 pathway.