The present invention relates to method of identifying whether or not an individual has Parkinsons disease (PD). In particular, the invention relates to a method for identifying whether or not an individual has PD in the pre-symptomatic phase of the disease or to distinguishing PD from another neurological disorder. The method of the invention comprises measuring the amount of soluble α-synuclein oligomers in a cerebrospinal fluid sample taken from an individual. The method optionally also comprises measuring the total amount of α-synuclein in the CSF sample, calculating the ratio of the amount of α-synuclein oligomers to the total amount of α-synuclein, and thereby determining whether or not the individual has PD. The method of the invention can be used in clinical trials to measure the effect of drugs in both PD animal models and human PD patients.
