An apparatus for measuring distensibility of a vessel, lumen or sphincter includes a balloon catheter having a balloon located adjacent a distal end of a catheter for inserting in, for example, the sphincter, the distensibility of which is to be determined. The balloon is inflated to respective first and second pressures to distend the sphincter to respective first and second transverse cross-sectional areas which are measured. The first and second pressures are selected so that the relationship between the pressure and the cross-sectional area of the sphincter is linear. The distensibility of the sphincter is determined as a distensibility index, which is the value of c from the equation of a line y=mx+c containing the values of the first and second pressures and areas, where x represents pressure and y represents area.