OBJECTIVE: To assess whether repeated administration of the Standardized Assessment of Concussion (SAC) and Balance Error Scoring System (BESS) demonstrates a practice effect in high school athletes. DESIGN AND SETTING: Subjects were assigned to either a control or practice group. Subjects in the control group were assessed twice, 30 days apart. Subjects in the practice group were assessed on 5 occasions. Four assessments were performed within a 7-day period and 1 more assessment 30 days after the initial test. SUBJECTS: Thirty-two uninjured high school athletes participated in this investigation. Sixteen were randomly assigned to a control group and 16 to a practice group. MEASUREMENTS: We measured performance on the SAC and on 6 test conditions of the BESS. RESULTS: We found a significant time-by-group interaction on BESS performance on the day-30 test session. A significant practice effect of the BESS was found during the course of repeated administrations in the practice group. After repeated testing, the number of BESS errors decreased with each test session, and error scores on day 5 (10.94 +/- 2.17) and day 7 (9.44 +/- 3.32) were significantly lower than the baseline score (12.88 +/- 3.34). We did not find group differences or a practice effect on the SAC (baseline score = 26.16). CONCLUSIONS: Our results revealed no practice effect with the SAC and a slight practice effect with repeated administrations of the BESS, especially with the single-leg stance on foam. Clinicians must acknowledge the potential for practice effects when readministering these concussion assessments to track recovery of an athlete or as a guide in return-to-play decision making.