The effects of vision on spatial and temporal characteristics of free unrestrained reaching movements of the arm were examined in 17 patients with ataxic syndromes due to degenerative disease of the cerebellum and its connections. Subjects were required to reach out and touch a visually presented target either in the dark or with the target and their finger visible. Overall, patients had prolonged reaction times and their movements were performed slower than normal. The spatial paths described by their fingertips were more circuitous, being of greater length than normal, a characteristic that was uninfluenced by visual conditions. Ataxic movements were less accurate than normal in two ways. First, there was greater spatial variability between repeat paths to the same target. The increased variability was present very early in the movement trajectory and at that stage was not influenced by visual feedback. Secondly, there were large constant errors at the end of movement, but only when moving in darkness. Patients with Friedreich's ataxia as well as those with intrinsic cerebellar degeneration showed the above abnormalities, although there were some quantitative differences between the two groups. We suggest these spatial errors arise because the cerebellum contributes either directly or indirectly to preparatory motor processes which, based on limb proprioceptive and retinal information, compute the pattern of muscle activity required to launch the limb accurately towards a target. Patients were largely successful at using visual guidance to make midflight adjustments to their movements in order to improve accuracy. This manifested as a reduction in spatial variability between repeat paths as the target was approached and a reduction in constant error. However, the visual correction mechanism did not appear normal. Under visual guidance, the end-phase of movement was often prolonged and characterized by excessive deviations or direction changes in the path. These deviations may be the expression of a visual guidance system producing corrections which themselves contain error requiring further correction. Thus, this process may be abnormal for the same reason that the initial pattern of muscle activity is misjudged.