Multimodality evoked potentials frequently reveal subclinical involvement of the central nervous system in patients with insulin-dependent diabetes mellitus. We devised this study to evaluate the possible effects of acute hyperglycaemia on visual evoked potential (VEP) parameters in type 1 diabetic patients. A hyperglycaemic clamp (250 mg/dl for 180 min) was performed in ten patients. Monocular pattern reversal VEPs (check size 15', contrast 50%) were recorded before, and every 30 min after the start of the clamp. Basal VEP latencies and amplitudes were normal bilaterally in nine patients. No significant changes in pattern reversal and flash VEP parameters were observed after the induction or during the clamp period. None of the neurophysiological parameters evaluated during the test was related to the duration of the disease, the basal VEP latency or amplitude or the presence of retinopathy. Our data suggest that the neurophysiological abnormalities detected in insulin-dependent diabetic patients are due to structural involvement of the central nervous pathways and not to functional damage induced by acute short-term hyperglycaemia.