Abstract: Disclosed are a system, apparatus, and method for determining a physiologic characteristic of a body lumen that include determining, at one or more selected pressures, the volume of incompressible medium infused into a balloon 54 of catheter 24 while the balloon is placed in each of (a) a lumen having a predetermined, fixed diameter and (b) a desired location of a body lumen having an unknown diameter. In certain variations, the physiologic characteristic (e.g., diameter, cross-sectional area) is determined by calculating the difference in the volume of infused medium between (a) and (b) at at least one static pressure. Other physiologic characteristics (e.g., compliance) are determined by calculating the difference in infused medium for (b) at at least two static pressures.