This study is a pilot trial to evaluate the effects of supported treadmill walking in patients with primary progressive multiple sclerosis (MS) on functional and psychosocial outcomes. Patients will exercise three times per week over the course of 6 months and will be evaluated at baseline, 12 weeks and 24 weeks into the program. Follow-up testing will also be conducted 12 weeks after participants have stopped exercising to determined the lasting effects of the intervention.