How is “Serious Mental Illness” Defined in Illinois?

Definition of Serious Mental Illness for Involuntary Treatment in Illinois

Video by Attorney Kevin O'Flaherty
Article written by Illinois Attorney Kevin O'Flaherty
Updated on
September 17, 2019

In this article, we explain the definition of serious mental illness for involuntary treatment in Illinois.  We answer the questions: “how is ‘serious mental illness’ defined in Illinois?”, “what illnesses are considered a ‘serious mental illness’ in Illinois?”, and “when is a person with a serious mental illness subject to involuntary admission to a mental health facility in Illinois?”

How is “Serious Mental Illness” Defined in Illinois?

In order for a mental illness to be considered “severe,” several conditions must be met.  Serious mental illness is defined by considering the diagnosis, disability and duration the afflicted has been experiencing symptoms.  There are specific disorders that are considered “serious” or “severe” mental illnesses, but having one of these disorders does not necessarily mean it should be defined as “serious.”

What Illnesses Are Considered a “Serious Mental Illness” in Illinois?

In Illinois, all illnesses listed in the most current edition of the Diagnostic and Statistical Manual (DSM), published by the American Psychiatric Association, are defined as serious.  This list includes, but is not limited to the following:

  • Schizophrenia
  • Paranoid and other psychotic disorders
  • Major depressive disorders (single episode or recurrent)
  • Bipolar disorders (hypomanic, maniac, depressive and mixed)
  • Schizoaffective disorders (bipolar or depressive)
  • Pervasive developmental disorders
  • Obsessive-compulsive disorders
  • Depression in childhood and adolescence
  • Panic disorder
  • Post-traumatic stress disorders (acute, chronic, or with delayed onset)
  • Eating disorders

You can find a complete list of disorders from the DSM by clicking here.

When is a Person with a Serious Mental Illness Subject to Involuntary Admission to a Mental Health Facility in Illinois?


Involuntary commitment and treatment for a serious mental illness is only used as a last resort for people who have been diagnosed and refuse care. There are many laws and procedures that must be followed to admit a patient against his or her will, and those procedures must be completed every 90 days to ensure the patient is reevaluated frequently. To qualify for involuntary treatment by court order, evidence must be brought before the court which proves that the respondent does indeed have a mental illness and poses an imminent threat to themselves or to others. If the patient is admitted in an emergency, he or she will go through the emergency certification process if deemed necessary by the attending physician. For more information, see our article entitled Involuntary Commitment to a Mental Health Facility in Illinois.

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