BACKGROUND: Younger age is being increasingly recognized as a risk factor for anterior cruciate ligament (ACL) graft rupture and contralateral ACL injury after ACL reconstruction. Recent reports estimate second ACL injury rates to be in the range of 20% to 40%, which is a significant concern and requires further exploration. PURPOSE: The primary purpose was to determine the rates of graft rupture and injury to the contralateral native ACL in younger athletes. We also sought to explore the influence of sex and age groupings (<18 years vs 18-19 years at the time of surgery) on the risk of subsequent ACL injury. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: The study cohort consisted of 354 consecutive patients who were younger than 20 years when they underwent their first primary hamstring tendon autograft ACL reconstruction. The number of subsequent ACL injuries (graft rupture or a contralateral injury to the native ACL) was determined at a mean follow-up of 5 years (range, 3-10 years). Subgroup analysis included sex and age (<18 years vs 18-19 years at the time of surgery) comparisons. Descriptive statistics were calculated for the timing of ACL graft rupture and contralateral ACL injury. RESULTS: The overall follow-up rate was 89% (316/354). Graft ruptures occurred in 57 patients (18%) at an average time of 1.8 years after surgery. Almost half (47%) occurred within the first postoperative year, and 74% occurred within the first 2 years. The highest graft rupture rate of 28.3% was in the youngest males (<18 years), and this was significantly higher than in females of the same age (12.9%), as well as in male patients older than 18 years (13.8%). Contralateral ACL injuries occurred in 56 patients (17.7%) at an average time of 3.7 years after surgery. There were no significant age- or sex-based differences for contralateral ACL injuries. The total number of patients who had at least 1 further ACL injury subsequent to the primary surgery was 110 (35%). CONCLUSION: The high rate of subsequent ACL injury in younger patients was confirmed. Early graft ruptures were more prevalent in patients who underwent surgery when they were younger than 18 years versus those in the 18- to 19-year age group. Males had higher rates of graft rupture than did females, with the youngest males (<18 years) at the highest risk in the investigated cohort.