OBJECTIVE This study was undertaken to quantify resting vaginal closure force (VCF(REST)), maximum vaginal closure force (VCF(MAX)), and augmentation of vaginal closure force augmentation (VCF(AUG)) when supine and standing and to determine whether the change in intra-abdominal pressure associated with change in posture accounts for differences in VCF. METHODS Thirty-nine asymptomatic, continent women were recruited to determine, when supine and standing, the vaginal closure force (eg, the force closing the vagina in the mid-sagittal plane) and bladder pressures at rest and at maximal voluntary contraction. VCF was measured with an instrumented vaginal speculum and bladder pressure was determined with a microtip catheter. VCF(REST) was the resting pelvic floor tone, and VCF(MAX) was the peak pelvic floor force during a maximal voluntary contraction. VCF(AUG) was the difference between VCF(MAX) and VCF(REST). T tests and Pearson correlation coefficients were used for analysis. RESULTS VCF(REST) when supine was 3.6 +/- 0.8 N and when standing was 6.9 +/- 1.5 N--a 92% difference (P < .001). The VCF(MAX) when supine was 7.5 +/- 2.9 N and when standing was 10.1 +/- 2.4 N--a 35% difference (P < .001). Bladder pressure when supine (10.5 +/- 4.7 cm H2O) was significantly less (P < .001) than when standing (31.0 +/- 6.4 cm H2O). The differences in bladder pressure when either supine or standing did not correlate with the corresponding differences in VCF at rest or at maximal voluntary contraction. The supine VCF(AUG) of 3.9 +/- 2.7 N, was significantly greater than the standing VCF(AUG) of 3.3+/-1.9 N. CONCLUSIONS With change in posture, vaginal closure force increases because of higher intra-abdominal pressure and greater resistance in the pelvic floor muscles.