Intramedullary nailing is widely used for the operative treatment of femoral fractures. Recently, the biologic healing of fractures has become better understood from fundamental investigations. However, there has been no clinical comparison between the fracture healing process with these two fixation methods. The purpose of this study was to use radiographs to compare callus formation with two types of intramedullary nailing for femoral shaft fractures: reamed interlocking (IL) nails and Ender nails. Femoral shaft type A fractures (AO classification) were studied. Twenty-seven fractures were treated with reamed IL nailing, and 81 fractures were treated with Ender nailing. The callus area was calculated from the maximum cross-sectional area on the anteroposterior and lateral radiographs. The callus appeared at a mean of 3.9 weeks after surgery in the IL group, and at a mean of 2.8 weeks in the Ender group ( P < 0.05). In the IL and Ender groups, fracture healing was noted at a mean of 3.4 and 2.0 months, respectively. The mean area of callus formation in the IL and Ender nailing groups was 439.5 mm(2) and 699.4 mm(2), respectively ( P < 0.02). Ender nailing results in abundant callus, which forms at an earlier stage after the procedure than in patients treated with IL nailing. Dynamization at the fracture site is reported to increase external callus formation. Our results indicate that the elasticity of the fixation obtained with Ender nailing promotes callus formation.