The visual-motor adaptation to lateral displacement of vision by prism glasses was studied in normal individuals and patients with cerebellar dysfunction, Parkinson's disease, right or left cerebral hemisphere lesions, Alzheimer's disease, or Korsakoff's syndrome. Adaptation was analyzed in two phases, the return to normal pointing with prism glasses in place (the "error reduction portion") and the mispointing in the opposite direction after the glasses were removed (the "negative aftereffect portion"). Negative aftereffect, which seems to be the best measure of true adaptation, was significantly reduced only for the cerebellar patients. This poor performance supports the involvement of the cerebellum in motor learning.