Huntington's disease (HD) is a neurodegenerative disorder characterized by motor, cognitive, and psychiatric symptoms and by a progressive degeneration of neurons in basal ganglia and in brain cortex. Brain-derived neurotrophic factor (BDNF) is a pro-survival factor for striatal neurons. Some evidence implicates a brain BDNF deficiency, related to mutated huntingtin expression, in the selective vulnerability of striatal neurons in HD. We compared BDNF serum levels in 42 patients with HD (range 28–72 years, mean age 51.9 ± 11.5), and 42 age-matched healthy subjects (range 25–68 years, mean age 48.2 ± 12.5). We evaluated the potential relationship between BDNF serum levels, CAG repeat number (range 40–54, mean 44.8 ± 3.4) and duration of illness (range 6–228 months, mean 103.6 ± 62.1). Serum BDNF levels were significantly lower in patients than in age-matched healthy subjects. Lower BDNF levels were associated with a longer CAG repeat length and a longer duration of illness. Severity of the illness, as assessed by the Unified Huntington's Disease Rating Scale (UHDRS) motor and cognitive scores, was negatively related to serum BDNF levels. These results in vivo confirm that the huntingtin mutation causes BDNF production to decline and show that the BDNF deficiency is detectable in HD patients' sera. Further studies on a larger sample size should confirm whether BDNF concentrations in patients' serum could be a useful clinical marker related to the patients' disease phenotype. © 2007 Wiley-Liss, Inc.