PURPOSE: To study which factors affect the deformity correction of vertebral body during kyphoplasty procedure. MATERIALS AND METHODS: 25 osteoporotic vertebral compression fractures were treated with balloon kyphoplasty from October 2006 to May 2007. Lateral radiographs were taken at 5 different stages with preoperative lateral decubitus position, after placing the patient in prone position on an operation table, after inflating balloon, after deflation and removal of the balloon, after inserting the cement. Then we analyzed the compression ratios and kyphotic angles of the vertebral bodies in each stage. RESULTS: Placing the patient in prone position showed significant postural reduction in kyphotic angle and restorement of the anterior and middle body height. The inflation of the balloon demonstrated significant reduction of kyphotic angle and restorement of the anterior and middle body height. After the deflation, anterior and middle body height has decreased significantly. After the deflation, the kyphotic angle and the anterior and middle body heights were not restored signigicantly compared with those of initial prone position. CONCLUSION: Vertebral height and kyphotic angle were partially recovered by inflating the balloon, but the correction was lost after deflating the balloon. Statistically, the body deformity was not restored significantly after deflating the balloon compared with that of intraoperative prone position. Therefore, we concluded that, in kyphoplasty of osteoporotic compression fractures, the postural reduction is the most important factor in deformity correction of fractured vertebral bodies.