Thirty-three patients who had been managed for an isolated, closed fracture of the femoral shaft when they were less than seventeen years old were examined at an average of thirty-three months (range, eighteen to fifty-six months) after the injury. Thirteen patients (39 per cent) had a persistent deficit in the strength of the quadriceps of the fractured limb, as identified on testing with a Cybex-II isokinetic dynamometer. Six patients (18 per cent) had a deficit according to the one-leg-hop for distance test, fourteen (42 per cent) had an average loss of ten millimeters in the circumference of the thigh, and sixteen (48 per cent) had an average loss of 10 degrees of flexion of the knee. The etiological factors that were thought to possibly be responsible for the weakness of the quadriceps were evaluated. The amount of maximum displacement of the fracture, as seen on the initial radiographs, was the only factor that was significant for the prediction of weakness of the quadriceps (p = 0.006) at both test speeds of the Cybex dynamometer and in all statistical analyses. Despite the persistent weakness of the quadriceps, none of the patients had a clinical problem at the latest follow-up examination. A subclinical deficit in the strength of the quadriceps may be related to damage sustained by the muscle at the time of the fracture. On the basis of the results of this study, we do not recommend a change from the traditional methods of treatment, which involve early application of a spica cast or use of traction followed by application of a spica cast.