Forty-four patients at risk for Huntington's disease and seventeen diagnosed as having the disease were studied longitudinally over a nine-year period to assess the evolution of cognitive impairment. Results showed that there were measurable changes in sensory-perceptual functions well in advance of the emergence of the movement disorder. Discrimination between patients with Huntington's disease and those at risk for the disease based on tests of sensory-perceptual functioning resulted in a correct classification rate of 95.1%. Subjects who had changed from being at risk to having the disease nearly all had discriminant scores from their first test sequence lying in either the disease range or the pre-symptomatic disease range. The present organic model of Huntington's disease is discussed in light of these findings and implications of the mathematical model underlying prediction are discussed.