The legal construct of competence to stand trial, or "adjudicative competence," is based on the premise that some mentally disordered defendants have impaired abilities when compared with most defendants and that adjudication should be barred if these competence-related abilities are significantly impaired. Where the line is drawn between sufficient and insufficient abilities has important consequences: as a result of being adjudicated incompetent, defendants may be detained and treated involuntarily and their trials will be delayed. However, no studies have systematically compared the capacities of relevant groups of defendants. In this study, 84 criminal defendants--42 of whom were hospitalized as incompetent and 42 of whom were regarded as unquestionably competent--were administered three instruments measuring capacity to understand legally relevant information. Incompetent defendants performed more poorly on all measures of understanding. Twenty-eight incompetent defendants were administered the measures a second time, after restoration to competence. Restored defendants improved their performance on all measures of understanding and their performance was similar to that of normal, competent defendants.