The present invention relates to method of identifying whether or not an individual has Parkinson's disease (PD). In particular, the invention relates to a method for identifying whether or not an individual has PD as opposed to another neurodegenerative disease. The method of the invention comprises measuring the concentration of a-synuclein (α-syn) and the concentration of unphosphorylated tau (tau) and/or phosphorylated tau (p-tau) in a cerebrospinal fluid sample taken from an individual. The method also comprises calculating the ratio of the concentration of tau and/or p-tau to the concentration of α-syn, and thereby determining whether or not the individual has PD.