The excitability of the motor cortex to magnetic stimulation was evaluated in seven patients with cerebellar lesions (six patients with a unilateral lesion) and in 20 control subjects. Magnetic motor threshold was defined at rest. In all but one of the patients with a hemicerebellar lesion the threshold was higher in the motor cortex contralateral to the impaired hemicerebellum and the right/left threshold asymmetry was clearly greater than normal. In the patient with a lesion involving both cerebellar hemispheres the magnetic threshold was above the normal limit on both sides. The latencies of motor responses were normal in all patients. This increase in the magnetic threshold of the motor cortex functionally related to the impaired hemicerebellum suggests the existence of a facilitating tonic action of the cerebellum on central motor circuits that might act at the cortical, or spinal level, or both.