Purpose/Hypothesis: Recent research has studied the influence of menstrual cycle hormones as a factor for the higher musculoskeletal injuries rates at the knee and ankle among women athletes compared to men. The fluctuation of hormone levels during menstrual cycle has been proposed to place females at high and low risk periods for injury due to the influence of hormones on proprioceptive input utilized to maintain postural control and generate effective motor outputs during movement. The purpose of this pilot study is to identify if the three phases of the menstrual cycle influence postural sway or balance responses during static and dynamic activities in women 18 to 28 years old. Number of Subjects: Females 18 - 28 years old (n 6) with normal 26 - 34 day menstrual cycle and no oral contraceptive use for past 3 months and no previous history of LE injury within the last 6 months or previous LE surgery. Materials/Methods: Subjects participated in a training session to use menstrual cycle tracking and self reporting procedures with Saliva Ovulation Q test and Urine Ovulation stick test (Mediplex)prior to balance testing. Subjects were randomly assigned to testing either during follicular phase (days 1 to 9), ovulatory phase (days 10-14) or luteal phase (days 15-28) of the menstrual cycle as determine by "ferning" pattern on dried samples of salvia and confirmed by color change of test band of urine. Subjects were subsequently tested on the next two phases and continued to track their menstrual cycle during balance testing. Subjects performed the six conditions of the Sensory Organization Test (SOT)protocol and the Functional Limitations Test(FLT) protocol; single leg stance, walking, tandem walk, step quick turn, step up and over and forward lunge on the Balance Master(R) system. Results: Descriptive statistics: means, standard deviation and ranges, were determined for equilibrium and composite scores of SOT and postural sway for FLT. ANOVA was employed for the effect of menstrual cycle phase on equilibrium and composite scores and postural sway responses. Pearsons' Correlation coefficient was employed to examine correlation of composite equilibrium score and postural sway score responses and anthropomorphic measures of height and weight. Conclusions: Differences were noted in the composite score of SOT during the luteal phase, when increased estradiol levels may reduce ligamentous strength. Differences were noted during the step up and over test during luteal phase and forward lunge test during the follicular phase, which is characterized by lower levels of estrogen. Clinical Relevance: While fluctuations of hormone during the menstrual cycle may place females at risk for injuries due to changes in postural control and balance performance, SOT or FLT may not identify individuals at risk for injury as these test evaluate postural and balance response that are anticipatory rather than reactionary. The use of salvia and urine sample tests are less invasive measure to determine menstrual cycle phases, but may not identify the distinct serum hormonal levels to assess risk for injury.