The most important mechanical characteristics of a nailing system are related to its stiffness (rigidity) and strength. This study evaluates the properties of three commercially available interlocking intramedullary nail systems using standardized test methods. An understanding of the mechanical properties along with the clinical data will assist the surgeon in choosing the optimum implant. Testing indicates that the bending strength and stiffness of the Grosse & Kempf, the AO/ASIF Universal, and the Russell-Taylor interlocking intramedullary nail designs are comparable. It is therefore not surprising that all of these nail systems have excellent clinical results. However, the nonslotted design is approximately 30 times more resistant to torsional loading than either the partially slotted design of the Grosse & Kempf nail or the fully slotted design of the AO/ASIF nail. The clinical relevance of the torsional values may not be known until a long-term comparison of the complication rates for these different systems is available. Analysis of screw design reveals a tradeoff in bending strength when compared to amount of bone purchase. The bending strength of fully threaded screws (allowing bicortical fixation) is less than that of partially threaded screws (allowing only unicortical fixation), which shows that for the implants tested, increased bone purchase requires a compromise in strength for similar sized screws.