Limited data exist concerning the mechanisms that underlie the different motor features of Parkinson's disease (PD) and their course over time. Our aims were (1) to identify longitudinal changes in PD patients and (2) to determine the neural correlates of the changes in movement initiation and velocity that occur in the course the disease. Thirteen early stage PD patients were scanned twice off antiparkinsonian medication with H(2)15O PET. Imaging was performed at baseline and again after 2 years while the subjects performed a motor task that was kinematically controlled across time. Paced reaching movements were made towards targets that were presented in a predictable order. Measures of movement onset time (OT) and mean velocity (MV) were recorded during PET. OT and MV decreased significantly from baseline to follow-up. With advancing disease, increasing subcortical activation was detected in the pallidum bilaterally and in the left putamen. In the cortex, motor-related activation increased in the right pre-SMA, anterior cingulate cortex and the left postcentral gyrus. Progressive delays in movement initiation (OT) correlated with increases in the right dorsal premotor cortex (dPMC). Slowing of movement (MV) was associated with declining activation in the left dorsolateral prefrontal cortex and dPMC. Our data suggest that with advancing PD, motor performance is associated with the recruitment of brain regions normally involved in the execution of more complex tasks.