Objectives1) To evaluate the test–retest reliability of lower extremity functional instability measures involving testing situations of varying complexity, and 2) To evaluate the interrelationships among performances observed during these tests and a maximal single-limb forward hop for distance.DesignA repeated measures design, repeated on two occasions.SettingPostural control laboratory.ParticipantsThirty young healthy subjects (23.5 ± 2.0 years).Main Outcome MeasuresSubjects performed single-limb standing balance and forward hop tests on two occasions completed within 1 week and at least 24 hours apart. Standing balance was assessed using a force platform and the following four progressively complex test situations: 1) standing on the stable platform with eyes open, 2) standing on a foam mat placed over the platform with eyes open, 3) standing on the stable platform with eyes closed, and 4) standing on the stable platform after landing from a maximal single-limb forward hop.Results and ConclusionsIntraclass correlation coefficients were moderate to excellent (0.41 to 0.91) suggesting that the standing balance tests are appropriate for distinguishing among group performances. Standard errors of measurement and associated 95% confidence intervals suggested that a change in an individual's standing balance performance of approximately 10–30% would be necessary in order to confidently state that a true change had occurred. Stronger relationships were observed between hop distance and standing balance tests performed with eyes closed (r= −0.63, p < 0.001) and after landing from a maximal hop (r= −0.53, p = 0.003), suggesting that tests that challenge postural control to a greater extent are more representative of functional performance.