PURPOSE: The purpose of this study was to compare the sliding distance of lag screw in patients with unstable femoral intertrochanteric fractures treated with intramedullary fixation using a cephalomedullary nail with a fixed angle between the neck and shaft of the femur in relation to reduction type by lateral radiographs. MATERIALS AND METHODS: Between January 2009 to October 2013, 86 cases (86 patients) with unstable femoral intertrochanteric fractures were treated with intramedullary fixation using a metal nail with a fixed neck-shaft angle and followed for at least 6 months. We used AO/OTA classification, and all cases were unstable fractures. Twenty cases were 31-A22, 54 cases were 31-A23, and 12 cases were 31-A3. There were 30 men and 56 women. Average patient age was 73.7 years (range, 47-97 years). We classified reduction types into three groups as postoperative lateral radiologic findings. Group 1 showed no displacement, group 2 showed anterior displacement of the femur neck, and group 3 showed posterior displacement of the femur neck. The radiological assessment compared the sliding distance of the lag screw between postoperative X-ray and last follow-up X-ray. RESULTS: Forty-two cases were in group 1, 22 cases were in group 2, and the other 22 cases were in group 3. There was no significant difference in the patient characteristics of each group. The sliding distances of the lag screw were 4.9±3.2 mm, 4.6±3.6 mm, and 8.5±4.9 mm, respectively, and group 3 showed a significant result (p<0.0001, p=0.024). CONCLUSION: In cases treated with intramedullary fixation using a cephalomedullary nail with a fixed neck-shaft angle, appropriate reduction with a lateral radiograph before screw fixation is needed to prevent excessive lag screw sliding.