The analysis of the molecular mechanisms involved in the initial interaction between neurons and Schwann cells is a key issue in understanding the myelination process. We recently identified Cthrc1 (Collagen triple helix repeat containing 1) as a gene upregulated in Schwann cells upon interaction with the axon. Cthrc1 encodes a secreted protein previously shown to be involved in migration and proliferation in different cell types. We performed a functional analysis of Cthrc1 in Schwann cells by loss- and gain-of-function approaches, using RNA interference knock-down in cell culture and a transgenic mouse line that overexpresses the gene. This work establishes that Cthrc1 enhances Schwann cell proliferation, but prevents myelination. In particular, time-course analysis of myelin formation in transgenic animals reveals that overexpression of Cthrc1 in Schwann cells leads to a delay in myelin formation, with cells maintaining a proliferative state. Our data therefore demonstrate that Cthrc1 plays a negative regulatory role, fine-tuning the onset of peripheral myelination.