Background The aim of this study was to measure the patient entrance skin dose (ESD) during percutaneous coronary intervention (PCI) for chronic total occlusion (CTO), and assess the factors that affect it. Methods and Results Radiosensitive indicators were used to measure ESDs during 23 procedures. Multiple regression analysis identified the strength of the linear relationship of the dependent variable (the natural logarithm of the maximum ESD) with the set of multiple independent variables (the natural logarithm of both the patient and angiographic data). The methods for estimating the maximum ESD and the estimated ESDs were compared with the actual ESDs. The average maximum ESD for all the patients was 2.7±1.5 Gy (median: 2.6 Gy). The natural logarithm of the maximum ESD correlated well with the natural logarithm of body mass index (BMI; p=0.0112), total fluoroscopic time (TFT; p=0.0002), and Frame Fixation Rate (p=0.0014). For the higher Frame Fixation Rate group, there were significant correlations between the BMI² × TFT value and maximum ESD (r=0.972, p<0.0001), and the TFT and maximum ESD (r=0.968, p<0.0001). There were no significant correlations for the lower Frame Fixation Rate group. Conclusions In PCI for CTO, the BMI² × TFT value or TFT is a good predictor of radiation skin injury risk, when the beam angulation is not changed frequently. It is important to change the beam angulation to control ESD during a prolonged procedure. (Circ J 2007; 71: 229 - 233)