Of 53 patients with unilateral, displaced and closed or grade 1 open tibial shaft fractures, 27 patients (group I) were randomized to treatment with an intramedullary nail and 26 patients (group II) to treatment with a plaster cast. 12 fractures in the latter group were considered stable enough for treatment with only a cast (group IIa), while 14 fractures in group II showed redisplacement during reduction under anesthesia or at 1 week follow-up. Therefore, these fractures were stabilized with cerclage or screws (group IIb), which was a prerequisite for continuing cast treatment. The mean time-to-union was 19 weeks for group I, and 25 weeks for group II. 6 patients in group I and 16 in group II had delayed union. The Nottingham Health Profile index scores on physical mobility, social isolation, work ability, and sexual life were significantly better in group I than in group II at 3 months after injury. Delayed union, malunion, and restricted range of motion at the ankle joint were common complications when these fractures were treated with a cast. We recommend intramedullary nailing for these fractures.