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Motor control was analysed by a visuomotor tracking movement using elbow flexion both in patients with cerebellar ataxia and in normal controls. A TV screen was divided into upper and lower halves, in each of which a vertical strip was displayed. The upper strip (T, target) was moved horizontally from the centre of the screen to the left or right by ramp voltage. The lower strip (D, displacement of the handle) was moved in proportion to angular displacement of the handle by a potentiometer coupled to the handle axis. The subject, while sitting in front of the TV screen, had to make D match the movement of T by controlling the handle with his right arm. The range of T movement was 30 deg in terms of the handle's angular movement. T velocity was 7.5, 15 or 30 deg/s. The subjects were told the direction and velocity of T in advance. The process of tracking was divided into three phases (initial catch-up phase, middle pursuit phase, and terminal phase), in each of which the performance of cerebellar ataxia patients differed from that of the controls. The characteristic features of the ataxic cases were (1) prolongation of the reaction time, mainly due to the increase of premotor time; (2) difficulty in selecting an appropriate amplitude of initial peak velocity in proportion to the target velocity in the initial catch-up phase; (3) disruption of smooth continuous movement, namely, the saccadic pattern in the middle pursuit phase; (4) delay in the initiation of deceleration in the terminal phase; (5) difficulty in corrective adjustment in reaching the final target point; and (6) irregular EMG activity in the agonist muscles and/or cocontraction of the antagonistic muscles. Quantitative treatment of the second and third features, as exemplified in the relationship between initial error and initial peak velocity and in the ratio of the movement arrest period, respectively, was found to be helpful in the evaluation of disease severity. The significance of these findings is discussed and the role of the cerebellar system in the control of slow voluntary movement is stressed.