Medical devices and methods for making and using medical devices are disclosed. An example medical device may include a guide extension catheter. The guide extension catheter may include a proximal shaft having a first outer diameter. A distal sheath may be attached to the proximal shaft and may have a second outer diameter greater than the first outer diameter. The distal sheath may be designed to extend past a coronary ostium and into a coronary artery so that another medical device can pass therethrough toward the coronary artery. An expandable balloon may be coupled to the distal sheath.