PURPOSE: To evaluate the causative factors in the catastrophic failure of LCP in the proximal humerus fracture. MATERIALS AND METHODS: Six patients (6 cases) were collected between October 2003 and July 2005. The mean age was 55.6 years (range: 38~70). The cause of injury was road traffic accident in four, fall down in one and slip down in one. According to the Neer classification, four were 2 part fractures, each one in 3 part fracture and 4 part fracture. RESULTS: Fixation failure occurred due to back-out of the plate and screw in five and plate breakage in one. Analysis of the preoperative radiographs revealed medial cortical defect in all and no bone graft and tension band wiring in the greater tuberosity fragment were carried out. Postoperative radiographs showed the anatomical reduction in three and non-anatomical in three. CONCLUSION: Non-anatomical reduction, insufficient medial bony buttress, inadequate screw length to the head and the neglect for the greater tuberosity fragment were the contributing factors to the failure of LCP. Knowledge of these factors will enable the surgeon to avoid failure of the LCP. Augmentation fixation and bone graft procedures with careful preoperative planning are necessary for successful fixation of LCP.