The cerebellum is certainly involved in fine coordination of movements, but has no efferences of its own to the muscles. Thus, it can exert its influence only via other cerebral areas that have those efferences. This study investigated in patients with cerebellar atrophy how cortical motor areas are affected by dysfunction of the cerebellum. The main question was whether the patients' slow cortical electroencephalogram (EEG) potentials during key-press preparation and execution would be generally altered or would be specifically altered when fine coordination was needed. In the coordination task, right- and left-hand keys had to be pressed simultaneously with different forces, under visual feedback. Control tasks were to press with both hands equally or with one hand only. The patients indeed had a performance deficit in the coordination task. Their cortical EEG potentials were already drastically reduced in the simple tasks, but were enhanced by the same amount as in healthy subjects when more coordination was needed. These results suggest that the cerebellum is not exclusively active in fine coordination, but is generally involved in any kind of preparatory and executive activity, whereas the motor cortex becomes more active with fine coordination. The role of the cerebellum might be to provide the motor cortex with information needed for coordinating movements. In cerebellar atrophy, this altered input may be sufficient for the motor cortex in controlling simple tasks, but not for complex ones.