Femoral overgrowth following femoral shaft fracture in children less then 13 years old has been documented by orthoroentgenogram in 74 patients from the time of healing 3 months following fracture until skeletal maturity. The femoral overgrowth averaged 0.92 centimeters (range 0.4-2.7) and was found to be independent of age, level of fracture, or position of fracture at the time of healing. The overgrowth was a universal phenomenon. Ipsilateral tibial overgrowth averaging 0.29 centimeters (0.1-0.5) occurred in 82 per cent of the patients. In the first 18 months following fracture 78 per cent of the overgrowth occurred. There was a time limit to the overgrowth phenomenon in 91 per cent of all patients following which the discrepancy persisted without change. By 18 months following fracture, however, only 12 per cent of the patients had completed the overgrowth and by 3 years and 6 months following fracture 85 per cent had completed their overgrowth. In 9 per cent of the patients overgrowth continued throughout the remaining growth period although at a slower rate than in the first 18 months following fracture.