X-linked recessive bulbospinal neuronopathy (X-BSN) is an adult-onset spinal and bulbar amyotrophy. Neurophysiologic studies demonstrate subclinical involvement of sensory nerves with diminished or absent sensory nerve action potientials and denervation changes, indicating the involvement of sensory neurons. We report the clinical features, findings of electrophysiologic study, and results of morphometric analysis of sural nerve pathology in a patient with X-BSN. Molecular genetic studies were also performed in the patient and his three daughters. Electrophysiologic studies revealed decreased amplitude sensory nerve action potentials and the presence of high amplitude motor unit potentials in all muscles tested. Sural nerve biopsy demonstrated axonal degeneration with a predominant loss of large myelinated fibers. Molecular genetic studies confirmed elongation of the CAG triplet repeats in exon 1 of the androgen receptor gene. Sequence analysis of the androgen receptor gene revealed that the number of CAG triplet repeats was 45 in the patient and was 45 to 48 in the mutant allele but only 19 to 30 in the normal allele in his three daughters. These findings suggest that both motor and sensory neurons are involved in X-BSN. Sural nerve biopsy and molecular genetic analysis are helpful in differentiation between X-BSN and other motor neuron diseases.