The authors compared the legal comprehension of 212 criminal defendants. These participants were divided according to their primary disorder: psychotic disorders (n=44), affective disorders (n=42), substance abuse disorders (n=54), and no diagnosed mental illness (n=72). The authors tested the defendants comprehension relative to their ability to stand trial and their competency to waive their interrogation rights. Participants were tested using several psychological measurement instruments. Results indicate criminal defendants with a diagnosed psychotic disorder were impaired in their abilities to understand legal proceedings when compared with non-psychotic defendants. These impairments clouded their understanding of their interrogation rights, their ability to communicate with counsel, the nature of the legal proceedings, and the possible outcomes of the proceedings. Interestingly, the authors also found that defendants without any mental illness demonstrated problems understanding their legal rights and the legal proceedings. In conclusion, the authors call on police and lawyers to identify those defendants with possible mental illness and to help protect their legal rights by referring these defendants for further evaluation. Furthermore, in addition to safeguarding the special needs of mentally ill defendants, more efforts should be made to educate all criminal defendants about their legal rights. Tables, references