Several studies have documented gender differences in anterior cruciate ligament injury (ACL) rates in athletes of all ages. In basketball, females have more than 3 times higher ACL injury rates than males. Most studies regarding ACL injury have examined college age athletes; few have involved growing children and teenagers. PURPOSE: To determine whether or not vertical ground reaction forces (vGRF) while landing onto a single leg play a role in explaining the gender difference in ACL injuries. METHODS: We recruited 41 players (22 females, 19 males) in 2 age groups (11–13 yr, 15–17 yr) from participants in an American Youth Basketball Tour national tournament. Leg dominance was determined by strength testing of the quadriceps and hamstrings. Participants hung from a high bar that was adjusted so that the lateral malleolus was 40 cm above the ground, then released the bar and landed on 1 foot onto an AMTI force platform. Each participant performed 3 drops on each leg. The 3-D ground reaction forces were sampled at 1200 Hz, and were processed using inhouse code written in Matlab 6.0, converting volts to forces in body weight units. Two way ANOVA design was used to evaluate for gender and age related differences in peak vGRFs and corresponding time to peak for dominant and nondominant legs. RESULTS: The data showed a significant age-gender interaction for peak vGRF during landing on the dominant leg (F = 6.278, p = .017). The 11–13 yr old females landed with higher peak vGRFs than males. This pattern reversed in the 15–17 yr old group. A similar but nonsignificant pattern occurred for the nondominant leg. No significant differences were noted for times to peak vGRFs. CONCLUSION: Among 15–17 yr olds, peak vGRFs were lower in females. Prior strength data analysis revealed that the same older females had weaker quadriceps relative to body weight than the males, which may partly account for peak vGRF differences. Lower vGRF likely indicates a softer landing produced by higher knee flexion. This may allow greater valgus and rotation at the knee, which have been shown to increase the load on the ACL. Such a combination may increase the risk for ACL injury. Supported by the Indian University President's Summer Undergraduate Research Initiative and the Quaker Oats Company.