The purpose of this study is to clarify the weak point in immature lumbar vertebrae associated with pedicle screw instrumentation. Ten immature thoracic and lumbar vertebrae were collected from calf spines. After installation of 6− and 7-mm-diameter pedicle screws into the pedicles of each specimen, pullout force was applied to the screw using the uniaxial MTS system until failure. Tightening torque during installation was measured. From the load-displacement curve, failure load was calculated and failure site was confirmed by radiographs. Inner pedicle diameters were measured after the pullout test, and percent fills of the pedicle screw were calculated. Mean tightening torque was 1.4 or 2.1 (Nm), mean failure load was 852.5 or 1,015.0 (N), and mean percent fill was 81.4 or 93.5% for 6− or 7-mm screws, respectively. Tightening torque and percent fill in 7-mm screws were significantly (p < 0.01) greater than that in 6-mm screws; however, failure load showed no significant difference (p = 0.10) between the two screw groups. Failure by screw pull-out occurred at the screw-bone interface or through the neurocentral synchondrosis (NS). NS fractures were observed in 20% of 6-mm screws, 60% of 7-mm screws, and 40% overall, whereas interface failures occurred in 80% of 6-mm screws, 40% of 7-mm screws, and 60% overall. In NS fracture group, tightening torque (p < 0.05) and percent fill (p < 0.01) were significantly greater than in the interface failure group. The results led us to conclude that the mechanism of the NS fracture is unclear. However, NS fracture could be one of the conceivable complications associated with pedicle screw fixation in the immature spine.