Female athletes are 4 to 6 times more likely to suffer an injury to their anterior cruciate ligament (ACL) compared to male athletes. Previous studies indicate that following ACL reconstruction (ACLR) surgery and rehabilitation, functional deficits persist in female athletes longer than males. Thus, it is important to identify sex specific tests that are sensitive to persistent deficits in females in order to determine readiness to return to sport. The purpose of this study was to determine if there was an effect of sex on the performance of predictive tests developed to identify functional deficits and determine readiness for athletes to return to sport following ACLR and rehabilitation. The hypothesis was that unilateral tests of functional ability would show deficits in the involved limb of females and not for males in athletes cleared for return to unrestricted sport participation. Seventeen females (17.1±1.3 yrs,166.5±5.3 cm,60.5±8.0 kg) and ten males (17.4±2.0 yrs,181.9±6.0 cm,98.0±17.6 kg) who returned to their sport within a year following ACLR were matched to control subjects (CTRL) based on sex, sport, age, mass, and height. Functional performance was tested in broad jump, vertical jump, long shuttle, pro shuttle, modified agility test (MAT), timed hop, triple hop, single hop and cross-over hop. A mixed-design repeated measures ANOVA(2X2) was used to test for the main effect and interactions of group (ACLR vs. CTRL) side (involved vs. uninvolved) on the dependant performance variables for both genders. There were no effects of limb involvement or group in either sex for the broad jump, vertical jump, long shuttle, pro shuttle, timed hop and MAT (p > 0.05), which indicated that athletic ability was similar between groups. Two single leg tests, triple hop and cross-over hop, showed a significant interaction and a decrease in the involved limb of ACLR (p < 0.05) for females, but not for males. Single hop showed a significant decrease in the involved limb for both males and females (p < 0.05). The study hypothesis was supported by the findings, as females with ACLR demonstrated decreased performance in unilateral tests in the involved limb, while males with ACLR did not, when both were compared to controls. These results indicate that unilateral hopping tasks should be utilized in females in order to identify functional deficits that should be targeted for further rehabilitation before full return to sport. As strength and conditioning professionals become more involved in the post ACLR rehabilitation process, understanding of objective criteria for measuring readiness to return to sports becomes of paramount importance. These findings indicate that sex differences exist in ACLR patients and should be considered to determine an athlete's ability to participate in unrestricted sport participation. Sports medicine professionals should emphasize correcting side to side deficits in female athletes during post ACLR rehabilitation and return to sport training. Acknowledgements: The authors would like to acknowledge funding support from National Institutes of Health Grant R01 AR049735 and R01 AR055563.