OBJECTIVE : To investigate the ideal suture material to test strain at nerve repair sites. Based on nerve strain tolerance, we aimed to determine which suture reliably failed by an average of 5% and a maximum of 8% strain when loaded to failure. METHODS : The median nerve of 19 cadavers was exposed in the distal forearm, transected proximally, and attached to a spring gauge. It was marked 5 cm on either side of its midpoint to measure strain. A laceration was created at its midpoint. We performed a tension-free end-to-end repair with a single epineural suture. Load to failure of the repair site was recorded. We recorded strain at failure and mode of failure (pullout vs breakage). Eight different sutures were tested: 6-0, 8-0, 9-0, and 10-0 nylon; and 6-0, 7-0, 8-0, and 10-0 polypropylene. RESULTS : Average strain at failure of 9-0 nylon most closely approximated 5% (4.9%). Moreover, 8-0 polypropylene and 10-0 nylon and polypropylene failed with average strains less than 5% and a maximum strain of failure less than 8%. Regardless of type, 6-0 to 8-0 caliber suture failed primarily by pullout of the suture from the epineurium whereas 9-0 and 10-0 nylon and polypropylene failed by suture breakage. Decreased precision through increased variability was seen when testing sutures failing via pullout. CONCLUSIONS : Nylon suture size 8-0 has been advocated as the suggested intraoperative aid to test strain at nerve repair sites. Our study suggests that 9-0 nylon may be a more appropriate testing suture because of its more predictable failure via breakage and its failure by a threshold of 5% to 8% strain. Although 8-0 nylon and polypropylene may also represent reasonable testing sutures, 8-0 nylon failed on average above 5% strain, with strains exceeding 8%, and both failed via the mechanism of pullout. CONCLUSIONS : This study's findings provide information for surgeons attempting to decide during surgery whether to perform direct nerve repair.