Motor evoked potentials (MEPs) were studied in 63 patients with disorders affecting central motor pathways. These were classified into five subgroups: motor neuron disease (16), multiple sclerosis (13), cerebral infarction (19), spinal cord lesions (10) and hereditary spastic paraplegia (5). Three patients with hysterical paraplegia were also studied. Results were compared to those obtained from 30 normal subjects. In normal subjects the mean central motor conduction time (CMCT) was 6.4 ms (range 3.4-8.1 ms, SD 1.0 ms) for abductor digiti minimi and 13.2 ms (range 9.7-17.0 ms, SD 2.3 ms) for tibialis anterior. Amplitude of the cortical MEPs was defined as a percentage of the size of the peripheral compound muscle action potential (CMAP) and ranged from 14 to 85%. Fifteen of 16 patients with motor neuron disease and all patients with cerebral infarction, multiple sclerosis (MS), spastic paraparesis and non-MS spinal lesions had abnormal studies including low amplitude, dispersed or absent responses and prolonged CMCTs. Patients with MS had markedly prolonged CMCTs. In three cases of hysterical weakness MEPs were within normal limits. MEPs are a useful method to detect pathology in the central motor pathways and may have a significant role in the diagnosis of disorders involving the upper motor neuron.