To clarify the role of the cerebellum in pursuit initiation (first 100 msec), we used infrared oculography to examine the effect of unilateral cerebellar lesions on the initial (0-20 msec) and later (80-100 msec) periods as well as the steady-state response (200-300 msec) of horizontal smooth pursuit in 10 patients with unilateral cerebellar lesions. These results were compared with those of 17 age-matched healthy subjects. Smooth pursuit was elicited with a step-ramp target movement with randomized horizontal directions and velocities of 10 degrees/sec and 30 degrees/sec. In the first 20-msec pursuit, velocity was 22% lower toward the side of the cerebellar lesion than away from it and 16% lower in the period 80 to 100 msec (normal differences, 2% and 3%). Later (200-300 msec), the ipsiversive/contraversive difference was smaller, but pursuit velocity in both directions was significantly lower in patients than in normals. No lesion affected the floccular region and/or the nodulus/uvula. Five lesions extended so far medially that they could have affected the medial deep cerebellar nucleus (fastigial nucleus). The remaining five were in the lateral hemisphere in areas previously considered uninvolved in pursuit generation. Our findings prove that the cerebellum participates in human pursuit initiation and that lesions in the lateral cerebellum possibly affect smooth pursuit.