We reviewed the data on 47 closed femoral shaft fractures in 46 children 10 years of age or less treated by early closed reduction and spica cast immobilization from 1980 through 1988. These children were followed at least through the time of fracture union, spica cast removal, and onset of unprotected weight-bearing. Treatment was defined as being primarily by spica cast immobilization if less than 7 days of skin traction preceded closed reduction and cast application. The mean age at injury was 4.4 years (range, 0.2-9.9 years). Mechanisms of injury were identified and segregated into those involving high-energy and low-energy trauma. Twelve of 23 fractures (52%) caused by high energy required at least one repeat closed reduction or other treatment to correct excessive shortening or angulation that occurred following the initial reduction. Four children required prolonged skeletal traction before reapplication of a spica cast. In contrast, only 2 of 24 fractures (8%) caused by low-energy trauma required repeat closed reduction and none required skeletal traction. Whereas the mean age of the children sustaining high-energy trauma (6.1 years) was higher than that of children in the low-energy trauma group (2.9 years), 7 of 12 fractures caused by high energy that required repeat reduction occurred in children 7 years of age or less. Femoral shaft fractures in children caused by high energy are more likely to become displaced following closed reduction and early spica cast immobilization than fractures caused by low energy. These children require careful radiographic monitoring following this type of treatment to assess fracture alignment.