The velocity of nerve conduction in myelinated axons is predicted to rise (up to a certain limit) with increasing distance between nodes of Ranvier, but little experimental data exist to support this prediction. Here, Wu et al. showed that young mice expressing mutant periaxin, which impairs Schwann cell elongation, exhibited abnormally short internodal lengths and a reduced axonal conduction velocity compared with wild-type mice. As the mutant mice aged, nerve conduction velocities increased — up to a maximum — with internodal length, confirming the prediction.