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To evaluate (1) systematic assessment of exercise tolerance in adolescents shortly after sport-related concussion (SRC) and (2) the prognostic utility of such assessment.Prospective randomized controlled trial.University and community sports medicine centers.Adolescents with SRC (1–9 days from injury). Sixty-five were randomized and 54 completed the study (mean age 15 years, 4 days after injury).Buffalo Concussion Treadmill Test (BCTT, n = 27) or not (controls, n = 27) on visit day #1. Heart rate threshold (HRt) at symptom exacerbation represented level of exercise tolerance. Participants reported symptoms daily for 14 days and then had follow-up BCTT (n = 54). Recovery was defined as returning to normal level of symptoms and exercise tolerance, verified by independent physician examination.Days to recovery and typical (≤21 days) versus prolonged recovery (>21 days). Mixed effects linear models and linear regression techniques examined symptom reports and time to recovery. Linear regression assessed the association of HRt with recovery time.Days to recovery (P = 0.7060) and typical versus prolonged recovery (P = 0.1195) were not significantly different between groups. Symptom severity scores decreased in both groups over 14 days (P < 0.0001), were similar (P = 0.2984), and did not significantly increase the day after the BCTT (P = 0.1960). Lower HRt on visit day #1 was strongly associated with prolonged recovery time (P = 0.0032).Systematic evaluation of exercise tolerance using the BCTT within 1 week after SRC did not affect recovery. The degree of early exercise intolerance after SRC was important for prognosis. This has implications for school academic and team preparation.