Walking impairment is a ubiquitous feature of multiple sclerosis (MS) and the O(2) cost of walking might quantify this dysfunction in mild MS. This paper examined the difference in O(2) cost of walking between persons with MS who have mild disability and healthy controls and the correlation between the O(2) cost of walking and disability. Study 1 included 18 persons with mild MS and 18 controls and indicated that the O(2) cost of walking was significantly higher in MS than controls and that disability was significantly associated with the O(2) cost of slow, moderate, and fast treadmill walking. Study 2 included 24 persons with mild MS and indicated that disability was significantly correlated with O(2) cost of comfortable, fast, and slow over-ground walking. We provide evidence that the O(2) cost of walking is an indicator of walking dysfunction in mildly disabled persons with MS and should be considered in clinical research and practice.