Abstract: A body lumen measuring instrument for insertion into a body passageway such as a bronchial tube or the like in order to determine the interior lumen diameter and/or axial length of the passageway at a predetermined location. This instrument can also be used to aid selection of a radioisotope capsule of proper size to be properly positioned at a specific location for tumor treatment. The measuring instrument includes a flexible sleeve having a handle secured to one end. A cable is slidably positioned within the sleeve and secured at one end to a slide actuator within the handle and at the other end terminates in a probe adapted for determining body passageway size under endoscopic observation. A measurement scale is associated with an indicator on the slide actuator in order to display the passageway size.