The loss of hydrostatic pressure that occurs as a person moves from the standing to the supine position causes a fluid redistribution that may confound the measurement of thigh cross-sectional area (CSA) if data are obtained while tissue fluid content is in flux. To determine the effects of changing postural position on thigh tissue CSA, mid-thigh axial scans of 13 older women were obtained at 5, 10 and 15 min of supine rest using computed tomography (CT). Scans were analysed for changes in CSA of subcutaneous fat (SF), low density muscle (LDM) and normal density muscle (NDM). A significant decrease from baseline was found in the CSA of NDM at 15 min [2.3+/-0.8 cm2 (+/-SE), 1.6%, P<0.05], with no change in LDM or SF CSA among any of the time intervals. The results of the current study suggest that potential measurement error can be minimized when baseline and follow-up CT-derived images of mid-thigh CSA are obtained within the first 10 min the subject assumes the supine position and that the CSA of NDM and LDM may be affected differently by supine rest.