Amyloid- (A) is known as the most prominent core protein in Alzheimers Disease (AD) senile plaques. Although research has focused mainly on A40 and A42 as potential cerebrospinal fluid (CSF) biomarkers, a range of A peptides with variable lengths has been demonstrated in the brains and CSF of AD patients. Recently, it has been found that the A43 peptide may be more abundant than previously assumed, could therefore play an important role in AD pathophysiology, and hence also function as putative biomarker. In this study the value of CSF A43 in AD diagnosis was investigated. A43 levels in CSF were highly correlated with A42 levels. Furthermore, in differentiation of AD from nondemented controls and from patients with Lewy body dementia and frontotemporal dementia, A43 had an equal diagnostic value as A42, both as a single biomarker and in combination with total and phosphorylated tau. In conclusion, quantification of A43 in CSF does not add novel diagnostic information to the differential diagnosis of AD compared to existing biomarkers.