After peripheral nerve injury, axons regenerate and become remyelinated by resident Schwann cells. However, myelin repair never results in the original myelin thickness, suggesting insufficient stimulation by neuronal growth factors. Upon testing this hypothesis, we found that axonal neuregulin-1 NRG1) type III and, unexpectedly, also NRG1 type I restored normal myelination when overexpressed in transgenic mice. This led to the observation that Wallerian degeneration induced de novo NRG1 type I expression in Schwann cells themselves. Mutant mice lacking a functional Nrg1 gene in Schwann cells are fully myelinated but exhibit impaired remyelination in adult life. We suggest a model in which loss of axonal contact triggers denervated Schwann cells to transiently express NRG1 as an autocrine/paracrine signal that promotes Schwann cell differentiation and remyelination.