To investigate the usefulness of motor evoked potentials (MEPs) in the diagnosis of neurological diseases and to correlate the presence of clinical signs of pyramidal tract involvement with abnormalities on MEPs we reviewed the records of all the patients that underwent electrophysiological study at our Departement between 1994 and 1995. We evaluated a total of 578 patients (487 with neurological diseases; 36 with hysterical paralysis; 55 with type 1 diabetes mellitus) that met the following criteria: 1) MEP study of the upper and lower limb muscles; 2) definite diagnosis supported by the appropriate tests. The overall incidence of central motor conduction abnormalities was 50.6% with an overall incidence of clinical signs of central motor pathways involvement of 41% (p < 0.001, Chi-square test). The percentage of MEP abnormalities ranged from 91.3% in cervical spondilotic myelopathy, 70.7% in multiple sclerosis and 76.2% in motor neuron disease to 0% observed in hysterical paralysis, radiculopathies and acquired cerebellar diseases. MEP abnormalities documented a subclinical involvement of central motor pathways in 13.8% of the patients whereas 3.8% of the patients presented pyramidal signs with normal MEPs. The higher percentages of subclinical abnormalities were documented by MEPs in patients affected by metabolic encephalopathies (45.8%), motor neuron disease (23.8%), diabetes mellitus (23.6%). The highest percentage of normal MEPs in the presence of pyramidal signs was found in cerebrovascular disease patients (24%).