Background: The extension catheter was originally developed to facilitate stent delivery to challenging lesions.We evaluated the efficacy and safety of using an extension catheter in patients undergoing percutaneous coronary interventions (PCI). Methods: Two interventional cardiologists reviewed the records of all consecutive patients who, between November 2011 and October 2015, had undergone PCI with a GuideLiner or Heartrail ST-01 extension catheter. Clinical demographics, vessel characteristics, procedural details, and outcomes were recorded. Results:We identified 136 (3.7%) eligible patients (male: 81.6%;mean age: 66.2  11.2 years) in 3665 PCI procedures. Seventy-two (52.9%) cases required increased support to cross severely calcified lesions. The remainder were coronary tortuosity [47 (34.6%)], chronic total occlusions [35 (25.7%)], previously deployed proximal stents [16 (11.8%)], and anomalous origin of coronary artery [9 (6.6%)]. There were 43 type B and 91 type C lesions. The success rate was 86.8% (118) and the complication rate was 6.6% (7 coronary dissections, 1 thrombus formation, and 1 stent dislodgement). All complications were successfully managed using endovascular interventions. The failure rate significantly (25.5%) increased if more than 3 of 6 peri-procedural factors coexisted: 1) long lesions (> 30 mm), 2) tortuosity, 3) calcification, 4) chronic total occlusion, 5) previous intervention history, and 6) previously deployed proximal stents. Conclusions: Using an extension catheter for challenging complex PCIs is safe and highly successful if the practitioner has adequate experience manipulating extension catheters.