CONTEXT: Sport specialization has been defined as year-round intensive training in a single sport to the exclusion of other sports. A commonly used survey tool created by Jayanthi et al, which classifies athletes as having a low, moderate, or high level of specialization, categorizes only athletes answering yes to "Have you quit other sports to focus on a main sport?" as highly specialized. We hypothesized that a measureable number of year-round, single-sport athletes have never played other sports and, therefore, may be inaccurately classified as moderately specialized when using this tool, even though most experts would agree they should be viewed as highly specialized. OBJECTIVE: To determine the proportion of athletes misclassified as moderately rather than highly specialized because they never played a previous sport. DESIGN: Cross-sectional study. SETTING: Hospital-based pediatric outpatient sports medicine clinic. PATIENTS OR OTHER PARTICIPANTS: Injured athletes aged 12 to 17 years who presented to the clinic between 2015 and 2017 and completed a sports-participation survey (n = 917). MAIN OUTCOME MEASURE(S): Sport-specialization level. RESULTS: Of 917 participants, 299 (32.6%) played a single sport more than 8 months per year, and 208/299 (69.6%) had previously quit other sports (highly specialized), whereas 91 (30.4%) had never played other sports (highly specialized and misclassified as moderate). Individual-sport athletes had a 2.03 times greater risk of being highly specialized and misclassified as moderate than team-sport athletes (relative risk = 2.03 [95% confidence interval = 1.37, 3.00]). Females had a 1.70 times greater risk of being misclassified as moderately specialized than males (relative risk 1.70 [95% confidence interval = 1.07, 2.70]). Of the 3 sports with the largest number of athletes, artistic gymnastics had the highest proportion (51.2%) who had never played other sports. CONCLUSIONS: The commonly used specialization survey misclassified a substantial number of highly specialized athletes as moderately specialized. Researchers should consider adding a fourth survey question, "Have you only ever played 1 sport?" to identify and better study this unique subset of misclassified athletes.