The aim of this study was to evaluate the influence of passive leg elevation and Trendelenburg position on the cross-sectional area (CSA) of the internal jugular (II) and subclavian veins (SCV). Ultrasound imaging was used for the following measurements of both the IJV and SCV baseline in the supine position (control); Trendelenburg position 15 degrees; reverse Trendelenburg position 15 degrees and passive leg elevation 50 degrees. Twenty healthy male volunteers were studied. Mean CSA of the IJV was 1.12 +/- 0.57 cm2 in control, 1.66 +/- 0.67 cm2 in the Trendelenburg position (P < 0.0001 vs. control), 0.38 +/- 0.23 cm2 in the reverse Trendelenburg position (P < 0.0001 vs. control), and 1.40 +/- 0.64 cm2 during passive leg elevation (P < 0.0001 vs. control). Mean CSA of the SCV was 0.92 +/- 0.23 cm2 in control, 0.98 +/- 0.17 cm2 in the Trendelenburg position, 0.86 +/- 0.21 cm2 in the reverse Trendelenburg position and 0.93 +/- 0.18 cm2 during passive leg elevation. The results indicate that passive leg elevation increases the CSA of the IJV, but has little effect on the SCV. The CSA of the IJV appears to be influenced more by gravitational factors than the SCV.