X-linked recessive bulbospinal muscular atrophy (BSMA) is an adult-onset form of motor neuron disease, of which androgen receptor (AR) gene mutations with increased size of a polymorphic tandem CAG repeat in the coding region was found by Fischbeck et al (1991). We investigated this AR gene abnormality by polymerase chain reaction (PCR) in 16 unrelated Japanese BSMA pedigrees, including 21 patients, 11 male siblings without any neurological signs and 9 female siblings. PCR products for AR-CAG repeat obtained from 21 affected individuals were enlarged in fragment size (about 100 bp longer than normal control size), whereas those from clinically unaffected brothers of the patients and their offsprings were all normal in size. Moreover, PCR products from 8 obligate heterozygous females (carriers) consisted of two different fragments with enlarged and normal size. Our results confirmed the findings reported by Fischbeck et al, and indicated that the detection of this AR gene mutations with increased size of a polymorphic tandem CAG repeat is beneficial for pre-onset diagnosis or carrier detection of this disease.