The effect of limb lengthening on neural structures was assessed with use of intraoperative neuromonitoring (IONM) during primary total hip arthroplasty (THA). The relationship between the critical limit of lengthening and anthropometric measurements was evaluated.Motor evoked potentials (MEPs) from the deep peroneal nerve (tibialis anterior muscle), tibial nerve (gastrocnemius muscle), and femoral nerve (quadriceps muscle), as well as somatosensory evoked potentials (SEPs) from the posterior tibial nerve, were recorded in 16 patients undergoing THA. Height, weight, the distance between the anterior superior iliac spine and the medial malleolus (ASIS-MM distance), and the total femoral length were measured preoperatively. Lower-extremity traction was performed after resection of the femoral head, and the amount of extremity lengthening was measured with use of an image intensifier. A maximum of 50% reduction in any one of the SEP or MEP amplitudes or a 10% increase in the SEP latency were considered to be indicative of the critical limit of lengthening.Initial IONM changes (indicating the safe limit of lengthening) and maximum allowed IONM changes (indicating the critical limit of lengthening) were reached in the deep peroneal nerve in all cases. The mean safe limit of lengthening (and standard deviation) was 14.9 ± 6.2 mm (3% relative to femoral length and 1.7% relative to ASIS-MM distance), whereas the critical limit of lengthening was exceeded at a mean of 22.4 ± 5.6 mm (5% relative to femoral length and 2.6% relative to ASIS-MM distance). When the critical limit was reached in the deep peroneal nerve, the mean decrease in MEP amplitudes was 27% (95% confidence interval [CI], 22.1% to 32.7%) for the tibial nerve and 12% (95% CI, 6.9% to 18.1%) for the femoral nerve. There was a positive correlation between critical lengthening and femoral length (r = 0.782; p < 0.001), ASIS-MM distance (r = 0.811; p < 0.001), and height (r = 0.835; p = 0.001). No correlation existed between the critical lengthening amount and the decrease in amplitude in the tibial and femoral nerves.The critical limit of nerve lengthening was directly correlated with anthropometric measurements. Nerve lengthening of 5% relative to femoral length and of 2.6% relative to ASIS-MM distance was found to be critical; however, these limits depended on the predetermined threshold values for IONM.Prognostic Level IV. See Instructions for Authors for a complete description of levels of evidence.