Disclosed is an antibody which specifically binds to the A&bgr(1-17) peptide and does not show substantial cross-reactivity against a A&bgr peptide selected from the group consisting of A&bgr(1-15), A&bgr(1-16), A&bgr(1-38), A&bgr(1-40), A&bgr(1-42) and a combination of one or more of them. Also disclosed are methods for diagnosing and monitoring Alzheimer&rsquos disease by detecting A&bgr(1-17) in a serum or blood cell sample.