The current study evaluated the discriminant validity and homogeneity of category fluency, Stroop interference, and planning measures of executive dysfunction and introduced a new measure of planning behavior. We compared performance of patients with schizophrenia, Parkinson's disease, and Huntington's disease, which are disorders associated with some impairment of executive function. Findings indicated that all three measures successfully discriminated between patient groups and controls. Correlational analysis suggested a strong association between test scores on these measures, which were only moderately related to performance on nonexecutive function measures. It is concluded that the three tasks are sensitive and relatively homogeneous in their recruitment of executive functions.