PURPOSE: To evaluate the scanning parameters affecting the apparent sizes of endoluminal lesions of thetracheobronchial tree, as seen on virtual bronchoscopy(VB), and to determine the optimal CT parameters fordemonstrating the real sizes of endobronchial lesions. MATERIALS AND METHODS: Spherical beads of 8 mm - 10 mmdiameter were randomly placed in the airways of fixed pig lung. CT scans were obtained with collimation and pitchof 3 mm/1, 3 mm/1.5, and 5mm/1, respec-tively. Volumetric data were reconstructed with 1mm-, 1.5 mm-, and2mm-collimation for each parameter. VBs were reconstructed with shaded-surface technique and soft tissuealgorithm. A 10mm-sized bead in the trachea and two 8 mm-sized beads in the left main bronchus were selected andtheir longest diameters were measured on VB at varying thresholds from -800 to -2 00HU. RESULTS: When themeasured diameters of beads on VB were recorded as the percentage of real sizes, they were 1) 78.9 %, 77.5%, and73.7% at collimations and pitches of 3 mm/1, 3mm/1.5, and 5 mm/1, respectively; 2) 77.9 %, 76.9 %, and 75.1 % at 1mm, 1.5 mm and 2 mm reconstructions, respectively; 3) 86.2 % / 83.4% / 80.4% / 77.0% / 74.8% / 70.2% / 64.5% atisosurface thresholds of -8 0 0 /-7 0 0 /-6 0 0 /-5 0 0 /-4 0 0 /-3 0 0 /-200HU, re-spectively; 4) 85.6 %, 75.0 %,69.3% at 23 mm, 17 mm and 11mm luminal diameters of lesion location, respectively. CONCLUSION: Overall, thediameters of endobronchial lesions are underestimated on VB. As the isosurface threshold values, collimations,pitches and reconstruction interuals decrease in size, the measured diameters approach to real diameter of thebeads. Beads in peripheral airways appear smaller than those in proximal airways.