Biomechanical strength and stiffness of 2 fixation treatments used to repair acute slipped capital femoral epiphysis were evaluated in bone specimens form immature dogs. A servohydraulic testing machine was used to create slipped capital femoral epiphysis in 7 pairs of femurs by shearing the capital femoral epiphysis along the physis in a craniocaudal direction. The slip was reduced and repaired with one 3.5-mm-diameter screw placed in lag fashion or 2 double-pointed, 1.6-mm (0.062 inch)-diameter smooth pins and retested. Strength and stiffness of each intact femur (which served as the control) and repaired femur were compared. Results of the study indicated that the failure strength of 2-pin fixation was significantly (P < 0.05) weaker than its control and the 1-screw fixation. There was no significant difference between failure strength of the 1-screw fixation and its control. The stiffness of 1-screw and 2-pin fixations was not significantly different, compared with each other, but was significantly (P < 0.05) less, compared with their respective controls.