Down's syndrome (DS) or trisomy 21 is the most common genetic cause of mental retardation, and adults with DS develop Alzheimer type of disease (AD). Cystathionine beta-synthase (CBS) is encoded on chromosome 21 and deficiency in its activity causes homocystinuria, the most common inborn error of sulfur amino acid metabolism and characterized by mental retardation and vascular disease. Here, we show that the levels of CBS in DS brains are approximately three times greater than those in the normal individuals. CBS is localized to astrocytes and those surrounding senile plaques in the brains of DS patients with AD. The over-expression of CBS may cause the developmental abnormality in cognition in DS children and that may lead to AD in DS adults.