OBJECTIVE: To compare characteristics and clinical outcomes of adolescent athletes with immediate and delayed onset of symptoms following sport-related concussion (SRC). A secondary objective was to describe the symptoms that are delayed and the timing of symptom onset. DESIGN: Prospective, repeated measures study. SETTING: Concussion Specialty Clinic. PARTICIPANTS: One hundred fifty-two participants (63.3% men) aged 11 to 24 (15.78 +/- 3.47) evaluated within 7 days (3.47 +/- 1.66) of sustaining an SRC. INDEPENDENT VARIABLE: Group classification of immediate (IMMEDIATE) or delayed onset of symptoms (DELAYED; >60 seconds following mechanism of injury) through structured clinical interview. MAIN OUTCOME MEASURES: Group comparisons on demographic and medical history factors, concussion and clinical profile symptom scores, computerized neurocognitive test scores, vestibular/oculomotor screening scores, and days to medical clearance. RESULTS: 24.3% of athletes in the sample were DELAYED. The groups did not differ on demographics and medical history. The DELAYED group had increased likelihood of posttraumatic migraine (PTM) as a primary/secondary clinical profile (P = 0.03). Groups did not differ on any other clinical profiles (P > 0.05). Groups did not differ on recovery time (P = 0.47), the IMMEDIATE group higher dizziness on vestibular/ocular motor screening (P = 0.016) and increased likelihood of dizziness being the initial symptom experienced (P < 0 .001). CONCLUSIONS: The results indicated that delayed onset of symptoms in athletes were relatively common following SRC, supporting continued evaluation of athletes for several days following suspected injuries. Delayed onset of symptoms was associated with PTM, whereas immediate onset was associated with more pronounced vestibular dysfunction, suggesting that clinicians should consider the timing of symptom onset when assessing and treating athletes following SRC.