Somatic sensation was studied in 23 patients from 14 families with hereditary spastic paraplegia. Quantitative sensory testing revealed significantly raised thermal, heat pain, vibratory and tactile thresholds in patients as compared with normal controls. Cutaneous sensitivity was more severely impaired in the feet than in the hand. All patients had at least one elevated sensory threshold. Sensory nerve conduction studies were abnormal in 6 patients, 5 of whom were members of the same family. Somatosensory evoked potentials were reduced on average to half the size of those of controls. Although the clinical picture is dominated by a spastic paraparesis, subclinical sensory impairment is common in hereditary spastic paraplegia and may reflect involvement of peripheral nerves, afferent pathways in the spinal cord or both.