Clinical and neurophysiological examinations were performed on seven patients with hereditary spastic paraplegia and on eight patients with primary lateral sclerosis. The results were compared with those obtained from a group of 39 control subjects. Prolonged latency times and decreased amplitudes of the posterior tibial nerve (PTN) somatosensory evoked potentials (SEPs) were found in the majority of the patients. The SEP changes occurred without sensory impairment or with loss of vibration sense only. There was no significant relation between the PTN SEP abnormalities and the severity of pyramidal signs for the whole patient group, nor longitudinally for the individual subjects. Analyses of PTN SEPs in patients suffering from slowly progressive spastic paraplegia (SP), therefore, seem to be a method to indicate a feature of spinal cord dysfunction that is not related to the severity of clinical signs. Considering the neuropathology of the spinal cord in SP patients, we furthermore argue that the ascending spinal pathway involved in conducting impulses for PTN SEPs probably uses other routes as well as the funiculus gracilis.