OBJECTIVES: To examine the safety and tolerability of an active rehabilitation program for adolescents who are slow to recover from a sport-related concussion, and secondarily to estimate the treatment effect for this intervention. DESIGN: Single-site, parallel, open-label, randomized controlled trial comparing treatment as usual (TAU) to TAU plus active rehabilitation. SETTING: Outpatient concussion clinic. PARTICIPANTS: Adolescents (N=19) aged 12 to 18 years with postconcussion symptoms lasting >/=1 month after a sports-related concussion. INTERVENTIONS: TAU consisted of symptom management and return-to-play advice, return-to-school facilitation, and physiatry consultation. The active rehabilitation program involved in-clinic subsymptom threshold aerobic training, coordination exercises, and visualization and imagery techniques with a physiotherapist (mean, 3.4 sessions) as well as a home exercise program, over 6 weeks. MAIN OUTCOME MEASURES: A blinded assessor systematically monitored for predetermined adverse events in weekly telephone calls over the 6-week intervention period. The treating physiotherapist also recorded in-clinic symptom exacerbations during aerobic training. The Post-Concussion Symptom Scale was the primary efficacy outcome. RESULTS: Nineteen participants were randomized, and none dropped out of the study. Of the 12 adverse events detected (6 in each group), 10 were symptom exacerbations from 1 weekly telephone assessment to the next, and 2 were emergency department visits. Four adverse events were referred to an external safety committee and deemed unrelated to the study procedures. In-clinic symptom exacerbations occurred in 30% (9/30) of aerobic training sessions, but resolved within 24 hours in all instances. In linear mixed modeling, active rehabilitation was associated with a greater reduction on the Post-Concussion Symptom Scale than TAU only. CONCLUSIONS: The results support the safety, tolerability, and potential efficacy of active rehabilitation for adolescents with persistent postconcussion symptoms.