Purpose: The aim of this study was to provide an in-depth analysis of the prevalence and consequences of within-sport specialization in track and field in the United Kingdom. Method: The competition histories of top 100 ranked athletes from four representative events (100 m, 800 m, long jump, and shot put) were recorded from a publically-accessible database. Athletes were drawn from Under 20 (U20), U15, and U13 populations from the 2014/15 season, U15 populations from the 2009/10 season, and U13 populations from the 2007/08 season. Athletes’ specialization status was defined based upon the number of event groups (sprint, endurance, jump, throw) in which they had recorded at least one performance. Chi-squared tests were used to examine the association between level of specialization at U13 and U15 and both performance and retention at subsequent age grades. Results: Within-sport specialization was rare among U13 and U15 track and field athletes, with approximately 10% of top 100 ranked U13s and 25% of top 100 ranked U15s competing in a single event group only. However, less than 35% of participants competed in sprinting, endurance running, jumping, and throwing events (i.e., diversification). There were no sex differences in the extent of specialization. Top ranked U20 female athletes were more likely to have diversified at U13 than their peers. There was no association between specialization at U13/U15 and subsequent retention. Conclusion: Administrators and coach educators should provide more sophisticated guidance for coaches and parents in relation to within-sport specialization.