In this prospective study of 34 patients with Parkinson s disease (PD), measurements of the short duration levodopa motor response have been performed every 3 years in defined off states. The mean time from initiation of levodopa treatment was 14.8 years, and 17 patients survived to the latest assessment stage. Off phase motor function worsened at a yearly rate of 2.2 of the maximum disability score. The magnitude of the levodopa response is well preserved as the disease progresses, and patients who developed motor fluctuations maintained better on phase motor function than nonfluctuators (P = 0.01). Ten patients, of whom 5 survive, developed dementia. There was no difference in pretreatment disability or initial levodopa response between demented and nondemented subjects. However, dementia was associated with worse on and off motor disability scores after 11 and 14 years (P <0.001), and a smaller levodopa response magnitude after 14 years (P = 0.008). The plot of sequential scores shows the association between cognitive decline and accelerating increase in motor disability. This suggests that the advanced phase of PD, when Lewy body pathology involves the cerebral cortex, progresses in an exponential rather than linear fashion. (c) 2009 Movement Disorder Society.