This study compared the legal abilities of defendants (N = 212) with current primary psychotic disorders (n = 44), affective disorders (n = 42), substance abuse disorders (n = 54), and no diagnosed major mental illness (n = 72). Defendants with primary psychotic disorders demonstrated more impairment than did other defendants in their understanding of interrogation rights, the nature and object of the proceedings, the possible consequences of proceedings, and their ability to communicate with counsel. Psychosis was of limited value as a predictor however, and high rates of legal impairment were found even in defendants with no diagnosed major mental illness. Sources of within-group variance were examined to further explain this finding. Policy and clinical implications of these results are discussed.