A method for diagnosing or prognosing Alzheimer's disease in a subject, or determining whether a subject is at increased risk of developing Alzheimer's disease, comprising: determining a level, or an activity, or both said level and said activity, of transcription product and/or a translation product of (i) a cystatin C gene or (ii) a polymorphic variant of a cystatin C gene in a sample from said subject;and comparing said level, or said activity, or both said level and said activity, of said transcription product and/or said translation product to a reference value representing a known disease or health status,thereby diagnosing or prognosing Alzheimer's disease in said subject, or determining whether said subject is at increased risk of developing Alzheimer's disease.