Mental health professionals are often called upon by the courts to provide expert opinion evidence to assist the court in dealing fairly with the accused person in the criminal justice system. Common law criteria for fitness to stand trial have evolved over 200 years but remain non-standardized, varying between countries, while sharing certain core characteristics. The courts most often agree with clinical assessments. This study examines all 1109 pretrial fitness assessments performed at the Forensic Unit of the Calgary General Hospital during the period 1991-1997 and measures the agreement between court decisions and the psychiatric opinions (99.8%). It contrasts this with the performance of instruments designed to measure fitness. It also examines the characteristics of the psychiatric reports, which adopt a semi-standardized format. Suggestions are made about the relative appropriateness of psychiatric reports and fitness instruments being used as expert evidence.