The authors designed a decision support system to assist mental health professionals to perform differential diagnoses of psychotic, mood, and organic mental disorders in accor dance with the American Psychiatric Association's revised third edition of the Diagnostic and Statistical Manual of Mental Disorders. A prototype system arrived at through a rigorous methodology illustrates a style of development that attempts to ensure system maintainability, correctness, and consistency of deduction and promotes high quality in software. Key words: mental disorders; decision support system; computer-assisted diagnosis; expert systems; artificial intelligence. (Med Decis Making 1993;13:43-48)