Managed care organizations (MCOs) use the concept of “medical necessity” to decide whether a prescribed treatment is warranted for a given medical condition. Because mental disorders lack the objective disease criteria common to medical illness, behavioral health administrators need a validated means to identify and quantify the severity of “medically important” aspects of mental disorders. The authors developed and tested a brief medical necessity scale for mental disorders in 205 patients presenting for initial evaluation. The scale had a factor structure with four subscales; good internal consistency, interrater reliability, and concurrent and predictive validity; and modest ability to identify patients requiring hospitalization and, in hospitalized patients, those requiring involuntary hospitalization. The authors propose use of the scale to better clarify decisions about level of care assignments and to better assess patient characteristics predictive of good outcome.