Studies on the variability of motor output in Parkinson's disease have found contrasting results depending on the speed-accuracy constraints of the task. The first goal of this study was to determine if Parkinson's disease subjects are more variable than control subjects. The second goal of the study was to examine the limitations on visual and motor processing that contribute to the changes in force variability in Parkinson's disease. Eight mild to moderate Parkinson's disease (age: 68-80 years) and eight matched control (age: 68-80 years) subjects maintained a constant level of force at 25% of their maximum voluntary contraction with their index finger and thumb (grip precision task) for 20 s while online visual feedback of the total force was viewed on a computer monitor. During the force task, subjects received visual feedback at varying frequencies. The sampled visual feedback levels were presented at intervals as slow as every 5 s to as fast as every 0.04 s (0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6 Hz). Force variability decreased over sampled visual feedback according to hyperbolic decay functions. The minimal visual processing time for both the Parkinson's disease and control subjects was approximately 160 ms. Motor output corrections were generated in both groups at a frequency of 1 Hz over a wide range of sampled visual feedback levels. However, the amplitude of the 1-2 Hz visuo-motor corrective process was amplified in Parkinson's disease, and this related to increases in force-output variability. The findings suggest that the basal ganglia are important for adjusting the amplitude of motor output at 1-2 Hz during visuo-motor feedback control.