Over the long term, Parkinson's disease (PD) appears to progress, in a linear fashion, at an annual rate of about 2% of the maximum motor disability score. This figure aligns quite well with pathological research on the rate that substantia nigra dopaminergic neurons are lost. An unexpected finding from cohort studies and clinical trials is that progression is twice as fast in prodromal PD, leading up to clinical diagnosis, and in recently diagnosed PD prior to the commencement of dopaminergic therapy. Levodopa initiation reduces motor disability by 40% of the pretreatment level. This benefit is composed of the short duration response, which is easily measured as the difference between and states, and the long duration response, which is comparable in size though not directly observable. Despite clinical impressions to the contrary, there is little evidence that the response to levodopa wanes over time or that axial motor deficits affecting speech, gait and balance become increasingly resistant to treatment. While not revealed by prospective longitudinal studies, the advanced PD phase, accompanied by visual hallucinations and cognitive decline, may show an exponential rate of change. Serial motor scale assessment, informed by a knowledge of symptomatic dopaminergic treatment effects, is probably still the best way to measure the underlying rate of progression of PD in clinical trials.