The aim of the study was to evaluate the effects of hyperglycemia on nerve conduction in patients with newly diagnosed diabetes mellitus type 1, and to investigate the significance of early electrophysiological diagnostics in these patients. The study included 85 newly disclosed patients with type 1 diabetes mellitus, in the first three months after the disease. Nerve conduction velocities (NCV) of further nerves were evaluated: median, peroneal, tibial and sural nerve as well as late responses (F-wave and H-reflex). Metabolic control parameters that were evaluated included: glycemia rate on the day of investigation and HbA1c. All patients had poor metabolic control parameters. We found NCV slowing predominantly in the tibial nerve (in 82.4% of patients). Prolonged F-wave latency was disclosed in 72.9% of patients, while H-reflex was evoked in 27.1% of patients only. The most sensitive parameter in the early neurophysiologic diagnostics was the measurement of F-wave latency. Our study underline the significance of early neurophysiological diagnosis, since hyperglycemia can play an acute role in NCV slowing, despite the absence of clinical symptoms, particularly in the first three months after the diagnosis has been confirmed.