Kennedy disease (KD) is an X-linked inherited motor neuron disease that is often accompanied by androgen insensitivity. Its estimated incidence in the US is approximately 1 case in 40,000 men. KD has also been reported in individuals of different racial backgrounds, especially in Japanese but the prevalence rate in Taiwan has not been fully investigated. Here we report a case of KD definitely diagnosed by abnormal expansion of a polymorphic tandem cytosine-adenine-guanine (CAG) triplet repeat in the first exon of the androgen receptor gene. The direct genotyping from polymerase chain reaction product is subsequently performed utilizing capillary electrophoresis. The patient's neurological conditions mimic amyotrophic lateral sclerosis (ALS). Since these two diseases have different etiologies and prognosis, it reminds us the necessity to rule out KD in face with a suspected male case of ALS.