People with multiple sclerosis (MS) are likely to benefit from regular exercise, but physical inactivity is more common among people with MS than among the general population. This small randomized study evaluated whether motivational interviewing (MI) affects adherence to and personal experience in an exercise program. Inactive people with MS participating in an eight-week exercise program were randomized to either three brief MI (n = 7) or three health coaching (n = 6) sessions. Session attendance for both conditions was high, and MI fidelity was rigorously and reliably measured using the Motivational Interviewing Treatment Integrity Scales. The feasibility of using this approach was demonstrated with a small sample. Large effects favoring the MI condition were found for physical exertion, affect during exercise, and fatigue, but no effects were found for adherence to the exercise program. Treatment integrity measures of MI were correlated with outcomes in expected directions. Although this study demonstrated the feasibility of this MI approach, the large effect sizes found should be viewed with substantial skepticism and replicated in sufficiently powered studies using objective measures of exercise adherence.