OBJECTIVE: To evaluate the efficacy and safety of a hip flexion assist orthosis (HFAO) in ambulatory patients with multiple sclerosis (MS). DESIGN: Fourteen week pre- and postintervention uncontrolled trial. SETTING: Outpatient rehabilitation clinic within an MS center. PARTICIPANTS: Ambulatory MS patients (N=21) with unilateral (or unilateral predominant) hip flexor weakness. INTERVENTION: Subjects were fitted with the HFAO on the weaker side, trained to use the device, and given a wear schedule. Subjects completed 2 baseline evaluations and follow-up testing at 8 and 12 weeks. MAIN OUTCOME MEASURES: Lower-extremity manual muscle testing, pain, and gait performance (Timed 25-Foot Walk, Timed Up & Go, 6-minute walk test, Mellen Center Gait Test). Subject satisfaction was evaluated by using a 9-item custom questionnaire. RESULTS: There was a statistically significant improvement of strength in the affected lower extremity at 8 and 12 weeks (effect size [ES]=0.63; ES=1.32, respectively), of pain at 12 weeks only (ES=-0.64), and of all gait tests at 8 and 12 weeks (ES range, 0.38-1.33). The overall mean satisfaction score at 12 weeks was 39 (maximum score, 45). No serious adverse events were recorded during the study. The most frequent side effect of the HFAO was low back pain (19%). No side effects led to discontinuation of the HFAO use during the study. CONCLUSIONS: The HFAO was safe and well tolerated. HFAO use was associated with significant improvement of gait performance as well as improvement of strength in the lower extremity fitted with the HFAO. Subjective reports suggest that there was an increase in daily life activity level.