Optic pathway glioma (OPG) is a childhood tumour of the visual pathway. Some OPG patients remain stable, whereas others experience rapid visual decline; however, conventional MRI cannot stratify these patients. We used multi-shell DTI to measure tumour invasion of the optic pathway in 23 OPG patients, in conjunction with visual assessment. A strong correlation was found between fractional anisotropy in the optic nerves and optic radiations, and visual acuity (p=0.00092 and p=0.008 respectively). Our study demonstrates that multi-shell diffusion-MRI offers increased sensitivity over conventional MRI in detecting white matter integrity and function in the visual pathway of OPG patients.