To retrospectively compare the longitudinal physical development of junior rugby league players between the Under 13 and 15 age categories in relation to their adult career attainment outcome.Retrospective longitudinal design.Fifty-one former junior rugby league players were retrospectively grouped according to their career attainment outcome as adults (i.e., amateur, academy or professional). As juniors, players undertook a physical testing battery on three consecutive annual occasions (Under 13s, 14s, 15s) including height, body mass, sum of four skinfolds, maturation, vertical jump, medicine ball chest throw, 10-60m sprint, agility 505 and estimated V˙O2max .Future professional players were younger than academy players with a greater estimated V˙O2max compared to amateur players. Between Under 13s and 15s, professional players (5.8±2.5cm) increased sitting height more than amateur (4.4±2.1cm) and academy (4.1±1.4cm) players. Logistic regression analyses demonstrated improvements in sitting height, 60m sprint, agility 505 and estimated V˙O2max between amateur and professional players with a high degree of accuracy (sensitivity=86.7%, specificity=91.7%).Findings demonstrate that the development of anthropometric, maturational and physical qualities in junior rugby league players aged between 13 and 15 years contributed to adulthood career attainment outcomes. Results suggest that age, maturity and size advantages, commonly observed in adolescent focused talent identification research and practice, may not be sensitive to changes in later stages of development in order to correctly identify career attainment. Practitioners should identify, monitor and develop physical qualities of adolescent rugby league players with long-term athlete development in mind.