INTRODUCTION : This research describes the therapeutic impact of hippotherapy on adolescents with cerebral palsy diplegia using the Physiological Cost Index and walking speed as outcome measures. METHOD : A single-system multiple-baseline design across subjects was implemented. Ten adolescents with diplegia were included. Participants had to be able to walk independently or with an assistive device. Adolescents were classified using the gross motor functional classification system levels I to III, and were 12 to 17 years old. Physiological Cost Index and walking speed were measured. Hippotherapy sessions took place once a week over a 12-week period. Each hippotherapy session (conducted by an occupational therapist) lasted 30 minutes. Average baseline measurements were compared with measurements taken before each intervention session and with post-intervention measurements. RESULTS : One participant’s Physiological Cost Index values decreased significantly (p = 0.014). Walking speed increased significantly over the course of treatment from the sixth intervention session (Wilcoxon Signed Rank Test, p = 0.024 post-intervention). The greatest improvement in walking speed was seen at intervention session 12 (p = 0.018). CONCLUSION : One participant’s Physiological Cost Index decreased and 10 participants showed increased walking speeds. Adolescents with diplegia need to adjust to increased growth and their walking ability may regress if therapy is not provided. Hippotherapy supports gross motor functioning and simulates the pelvic movement of normal walking.