ObjectiveTo identify a significant change in the laxity of the anterior cruciate ligament (ACL) in the competitive adolescent female athlete throughout the different phases of the menstrual cycle.DesignProspective, single-blinded 8-week study set during a winter sports season.SettingSuburban Ohio Division I high school.Participants26 members of gymnastics, soccer, track, tennis, and basketball teams. All participants were screened for normal menstrual cycles (26–30 days, menses 4–7 days long).Main Outcome MeasuresKT-1000 arthrometer was used to measure laxity by performing repeated measures throughout an 8-week period. Measurements were taken before the athletes' workouts. The athlete charted the menstrual periods on a monthly calendar. The measurements were then grouped into the three phases of the menstrual cycle (follicular, ovulatory, and luteal) and averaged.ResultsRight knee laxity measured 4.98 mm follicular phase, 5.24 mm ovulatory, and 5.09 mm luteal. Left knee laxity measured 4.51 mm follicular, 4.43 mm ovulatory, and 4.62 mm luteal. There was no statistical difference among the three phases in the left (p = 0.9) and right (p = 0.7977). Additionally, left ACL laxity was significantly less in all three phases. We found no statistically significant variability in laxity among the five sports sampled (p > 0.63 to 0.10) and different ages (p = 0.404)ConclusionsWe found an insignificant change in ACL laxity from follicular to luteal phases of the menstrual cycle. This indicates that no single phase of the menstrual cycle clinically affects the ACL more than the next. Although the presence of sex hormones—particularly estrogen—may indeed predispose females to higher ACL injury rates, we did not find any evidence that hormonal level changes equate with significant ACL laxity changes. We conclude that the menstrual cycle does not significantly affect ACL laxity in the competitive adolescent female athlete.