BACKGROUND AND OBJECTIVES: The cerebellar lesion causes an initiation deficit of smooth-pursuit eye movement depending on the location of the lesion. We investigated the initiation of smooth pursuit in patients with cerebellar infarction and in healthy subjects, using step-ramp stimuli. MATERIALS AND METHODS: Ten patients with cerebellar infarction documented by brain magnetic resonance imaging and fifty healthy subjects are recruited. To estimate the initiation of smooth pursuit, the onset latency and initial acceleration during the first 100ms of the horizontal smooth pursuit were estimated using the step-ramp target stimuli (5degrees/sec, 10degrees/sec, and 20degrees/sec). RESULTS: In healthy subjects, onset latency of pursuit was shortened and initial acceleration was increased as target velocity was increasing. In patients with unilateral cerebellar infarction, the onset latency of ipsilesional smooth pursuit was significantly delayed at the target velocities of 10degrees/sec and 20degrees/sec. For the fast target velocity of 20degrees/sec, there was significant decrease of the initial acceleration of contralesional pursuit. CONCLUSION: In comparison with the healthy subjects, the patients with unilateral cerebellar lesions showed significant delay of pursuit onset and decrease of initial eye acceleration in the fast target velocity. These results support that the cerebellar lesions affect not only steady-state smooth pursuit gain but also the processing time required to initiate smooth pursuit, i.e., onset latency and initial acceleration. More extensive study is needed to confirm the role of cerebellum for parametric adjustment of each component of smooth pursuit.