Cerebellar dysfunction is a common feature of multiple sclerosis (MS), leading to disabling symptoms such as tremor. In MS, brain atrophy is the most accepted correlate of neurodegeneration; however cerebellar atrophy does not seem to correlate with the degree of cerebellar dysfunction. Here we explore axonal degeneration, a key driver of disability, with a fibre-specific marker based on diffusion-weighted MRI – fibre density and fibre bundle cross-section. We found that loss of cerebellar fibre density and fibre cross-section was associated with increased clinical cerebellar dysfunction. Fibre-specific measures could provide a useful marker of cerebellar dysfunction in MS.