Background. Although the success rate of PCI for CTO is yet to reach a satisfactory level, prognoses have been improved in successful cases. The greatest challenge is the passage of the guidewire and a variety of new approaches including a parallel wire technique and a retrograde approach have been reported. However, the fundamental question of what type of guidewire is the most appropriate as a first-choice guidewire has not been answered.Tapered guidewires have recently been reported to be useful for CTO lesions. In addition, a multicenter prospective registry indicates the effectiveness of 0.010-inch guidewires (PIKACHU registry, personal communication). Its mechanism is associated with micro-channels ranging from 100 to 300 μm in size in CTO lesions. If 250 μm micro-channels are present in 60% of CTO lesions, a 0.010-inch (equivalent to 250 μm) guidewire theoretically can pass through the lesion at the probability of 60%, and this assumption consists with the findings of the PIKACHU study. This idea also suggests that a 0.014-inch (350 μm) guidewire is unlikely to pass through lesions.