We determined the concentrations of free homocysteine (HC) and total HC in the cerebrospinal fluid (CSF) of patients with Alzheimer's disease (AD) or Parkinson's disease (PD) in order to elucidate whether HC is related to the pathogenesis of these neurodegenerative diseases. The concentration of free HC did not differ significantly from that of the normal controls, while the concentration of total HC was significantly higher in the AD and PD patients (+31% in AD,+31% in PD; p<0.05). These findings suggest that an increase of total HC concentration in the brain is commonly seen in patients with AD and PD, and this may be related to the pathogenesis of these two diseases.