X-linked recessive bulbospinal neuronopathy is a motoneuron disorder to be distinguished from amyotrophic lateral sclerosis, Effective treatment is not known. Patients with X-linked recessive bulbospinal neuronopathy may show gynecomastia and testicular atrophy, and a mutation in the androgen receptor gene has been found associated with the disease. Intermediate steps leading from the androgen receptor abnormality to the clinical syndrome have not yet been elucidated. Therefore, binding of androgen ([3H]dihydrotestosterone) to its specific receptor by genital skin fibroblasts cultured from a patient with X-linked recessive bulbospinal neuronopathy and confirmed androgen receptor mutation was studied. Markedly decreased binding capacity was found. We treated the patient for 6 months with nandrolone-decanoate. No effect on his neuromuscular status was observed during 2 years of follow-up.