INTRODUCTION: Inadequate sample size and power in randomized trials can result in misleading findings. This study demonstrates the effect of sample size in a large clinical trial by evaluating the results of the Study to Prospectively evaluate Reamed Intramedullary Nails in Patients with Tibial fractures (SPRINT) trial as it progressed. METHODS: The SPRINT trial evaluated reamed versus unreamed nailing of the tibia in 1226 patients, and in open and closed fracture subgroups (N = 400 and N = 826, respectively). We analyzed the reoperation rates and relative risk comparing treatment groups at 50, 100, and then increments of 100 patients up to the final sample size. Results at various enrollments were compared with the final SPRINT findings. RESULTS: In the final analysis, there was a statistically significant decreased risk of reoperation with reamed nails for closed fractures (relative risk reduction 35%). Results for the first 35 patients enrolled suggested that reamed nails increased the risk of reoperation in closed fractures by 165%. Only after 543 patients with closed fractures were enrolled did the results reflect the final advantage for reamed nails in this subgroup. Similarly, the trend toward an increased risk of reoperation for open fractures (23%) was not seen until 62 patients with open fractures were enrolled. CONCLUSIONS: Our findings highlight the risk of conducting a trial with insufficient sample size and power. Such studies are not only at risk of missing true effects but also of giving misleading results.