Disclosed is an expandable transluminal sheath 1400, for introduction into the body while in a first, low cross-sectional area configuration, and subsequent expansion of at least a part of the distal end 1404 of the sheath 1400 to a second, enlarged cross-sectional configuration. The sheath 1400 is configured for use in the vascular system and has utility in the performance of procedures in the left atrium. The access route is through the inferior vena cava to the right atrium, where a trans- septal puncture, followed by advancement of the catheter is completed. The distal end 1404 of the sheath 1400 is maintained in the first, low cross-sectional configuration during advancement to the right atrium and through the atrial septum into the left atrium. The distal end 1404 of the sheath 1400 is subsequently expanded using a radial dilatation device 1600. In an exemplary application, the sheath 1400 is utilized to provide access for a diagnostic or therapeutic procedure such as electrophysiological mapping of the heart, radio-frequency ablation of left atrial tissue, placement of left atrial implants, mitral valve repair, or the like.