PD is a progressive neurodegenerative illness that affects about 50.000 people in the Netherlands. Diagnosis is based on clinical criteria. However, purely on clinical grounds, especially in the early stage, it is not possible to differentiate PD from other parkinsonian syndromes like multiple system atrophy, Progressive Supranuclear Palsy, vascular parkinsonism, drug induced parkinsonism and essential tremor. Accurate differentiation is important because treatment and prognosis varies between the different syndromes.