Abstract: The present invention relates to a new method for the in vitro diagnosis of a neurological disorder, more particularly of Alzheimer's disease (AD) and of mild cognitive impairment (MCI). The present invention also relates to the use of orexin-A (hypocretin-1), a specific fragment thereof, a nucleotide encoding the same or a combination thereof, as a biomarker for the in vitro diagnosis of a neurological disorder, more particularly of Alzheimer's disease (AD), alone or in combination with other known bio markers for the diagnosis of AD. The present invention finally relates to kits for the in vitro diagnosis of a neurological disorder, more particularly of AD, comprising means for the detection of orexin-A.