ABSTRACT Introduction: In young patients with mononeuropathy who lack family history and precipitating factors, hereditary neuropathy with liability to pressure palsy (HNPP) may be a possibility. Our objective is to propose neurophysiological criteria for HNPP in patients <30 years of age. Methods: We conducted a national multicenter retrospective clinical and neurophysiological study in patients under 30 with genetically confirmed HNPP. Results: All of the 51 patients included in the study had at least 1 demyelinating pattern in 2 asymptomatic nerves, and 3 abnormalities were found in almost 90%, including slowed motor nerve conduction velocity across the elbow in at least 1 ulnar nerve (97.5%), increased distal motor latency (DML) in at least 1 fibular nerve (95.8%), and increased DML in both median nerves (89%). Age influenced DML slightly only in the fibular nerve. Discussion: Dissemination of nerve involvement in HNPP incites to perform a complete nerve conduction study. including bilateral ulnar, fibular, and median nerves. Muscle Nerve 57: 217–221, 2018