Devices and methods for accurately determining the size of an ostium of a patient and in particular sinus ostium. Methods for measuring a target ostium comprise inserting the distal end portion of an ostium measuring device into a patient, locating the target ostium with the measuring device, positioning the distal end portion of the measuring device appropriately adjacent to or into the target ostium and determining the diameter(s) of the target ostium by comparing the length, diameter and/or the circumference of the portion of the distal end portion of the measuring device adjacent to or within the target ostium.