Plasma levels of testosterone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) after 3 or 6 days of administration of the synthetic androgenic hormone fluoxymesterone (10 mg/day) were measured in 26 patients with X-linked recessive bulbospinal neuronopathy (X-BSNP) and 22 age-matched male controls. The testosterone, LH and FSH levels in the controls were markedly suppressed after administration, but in the patients with X-BSNP, they were suppressed significantly less. The level of suppression varied considerably with the patients, and those of plasma testosterone and FSH were significantly correlated with the number of CAG repeats in the androgen receptor gene. These findings suggest that the androgen action was aberrantly transduced in the target organs in the patients with X-BSNP and which is related to the elongated CAG repeat in the androgen receptor gene.