A study of ninety-nine patients who had a unilateral, displaced, isolated closed fracture of the tibial shaft was performed to determine the effect of the type of treatment on the clinical outcome. Forty-seven patients were managed with closed intramedullary nailing with reaming, and fifty-two were managed with closed reduction and a cast. The two groups were comparable with regard to the ages of the patients, the locations and amounts of displacement of the fractures, and the number of patients who had a history of smoking. The time to union was shorter in the patients who had been managed with intramedullary nailing than in those who had been managed with a cast (mean, eighteen compared with twenty-six weeks; p = 0.02). A non-union occurred in one patient (2 per cent) who had been managed with nailing and in five patients (10 per cent) who had been managed with a cast. There were no infections in either group. Removal of the nail was performed electively in twenty-six patients.Twenty-five patients who had been managed with nailing and twenty-five who had been managed with a cast were followed for a mean of 4.4 years. With use of the Iowa Knee Evaluation and the Ankle-Evaluation Rating System, the patients who had had nailing had mean scores of 96 points (range, 68 to 100 points) and 97 points (range, 74 to 100 points) for the knee and the ankle, respectively, compared with 89 points (range, 62 to 100 points) and 84 points (range, 62 to 100 points) for those who had been managed with a cast (p < 0.05). Administration of the Medical Outcomes Study Short Form-36 Health Survey to the twenty-five matched pairs of patients yielded scores that were significantly better after nailing than after treatment with a cast (a mean of 85 points [range, 27 to 99 points] compared with a mean of 74 points [range, 20 to 97 points]; p < 0.05).We concluded that the treatment of displaced closed fractures of the tibial shaft with closed intramedullary nailing with reaming provides functional results that are superior to those obtained with use of a cast.