The carbohydrate-deficient glycoprotein (CDG) syndromes are multisystemic disorders involving the glycosylation pathway. The most common subtype is CDG syndrome type I (CDG I). In most CDG I patients a phosphomannomutase (PMM) deficiency has been recognized as the basic defect. We made a neurophysiological evaluation in an 8-year-old boy affected by CDG I with PMM deficiency. The evaluation included central and peripheral nervous system assessment [electroencephalogram (EEG), multimodal evoked potentials (MEP), somatosensory evoked potentials (SEP), visual evoked potentials (VEP), auditory brainstem response (ABR), electroretinogram (ERG) and motor and sensory nervous conduction velocity (NCV)]. We found a peculiar electrophysiological pattern characterized by slowly and mildly progressive motor NCV reduction; progressive impairment of ERG and VEP; slowing of background activity and sharp waves at the EEGs; late sensorineural abnormality of ABR; decreased amplitude and increased latency of SEP. To our knowledge this is the first report involving the neurophysiological aspects both at onset and during follow-up of a case of CDG I with proven PMM deficiency.