Objective-This study investigated the diagnostic and prognostic value of the King-Devick (K-D) test in conjunction with treadmill testing in adolescents after sport-related concussion (SRC) in an outpatient concussion management clinic without baseline measures. Design-Prospective cohort.Methods-The K-D test was administered pre-and post-exercise on a graded treadmill test to acutely concussed (AC, <10 days from injury, n=46, 15.4±2.1 years) participants for 2 clinic visits (1 week apart) and to matched controls (MC, n= 30, 15.8±1.4 years) for 2 visits (1 week apart). Initial K-D test times were compared between MC and AC. Changes in times from pre-to postexercise during a treadmill test were compared for MC and AC and from Visit 1 to Visit 2. Smooth pursuits and repetitive saccades were compared with initial visit K-D test performance.Results-Comparison of pre-exercise K-D test times at Visit 1 distinguished MC from AC (46.1±9.2s vs. 53.7±13.0s, p=0.007). Comparison of pre-and post-exercise K-D test times revealed significant improvements for MC (46.1±9.2s vs. 43.1±8.5s, p<0.001) and AC who recovered by Visit 2 (Fast Recovery Group [FRG], n=23, 50.4±10.0s vs. 47.3±9.8s, p=0.002). No significant difference was seen in pre-and post-exercise K-D test times on Visit 1 for AC who took longer than 2 weeks to recover (Slow Recovery Group [SRG], n=23, 57.0±15.0s vs. 56.0±16.3s, p=0.478). At Visit 1, AC had more abnormal smooth pursuits than MC (17% vs. 3%, *