This study examined pathological associations and dissociations of functional cognitive systems in patients with multiple sclerosis and Huntington's disease. Using the subtests of the WAIS-R, two motor tests, and the word fluency test, the interest correlations showed distinct patterns. In comparison to normals, the two clinical groups exhibited a greater degree of association among the tests. Subsequently, word fluency performance was predicted from these tests. For the normals, the overall predictive power was quite low (7%). For the MS group, the predictive power rose to 28%. For the HD group, the predictive power was 50%. These results suggest that pathological association of functional systems may be a marker of brain dysfunction and that the affected systems may be delineated by these methods.