A quantitative assessment of smooth pursuit eye movements was performed in patients with cerebellar ataxia and Parkinson's disease, using sinusoidal stimuli which covered a wide range of frequencies and velocities. Predictive strategies were also studied with pseudo-random stimuli whose characteristics could be varied to produce both predictable and unpredictable waveforms. The cerebellar group demonstrated reduced slow-phase eye-velocity gain for all target frequencies and velocities, reflecting dysfunction in the closed-loop, visual-feedback pathways. In contrast, reduced gain in the Parkinson's disease group could only be demonstrated at frequencies at or above 1.2 Hz during sinusoidal target motion. This was attributed to an impaired ability to generate saccades during the tracking task, as evidenced by the changes in overall eye displacement. During pursuit of pseudo-random target motion, the magnitude of breakdown in the smooth pursuit response as the target motion was made less predictable, was similar in both the patient groups and the controls, indicating that the predictive mechanisms were intact in these subjects.