Background: The current classification of patients with New York Heart Association Class IV symptoms does not offer adequate description to allow optimal selection of patients for the current options of medical and pacing therapies, cardiac transplantation and mechanical circulatory support. Methods: Seven clinical profiles and an arrhythmia modifier were developed and implemented into the first year of data collection for the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS). The INTERMACS Coordinators' Council provided ongoing feedback regarding the characterization of patients receiving implantable devices. Results: The definition of 7 clinical profiles revealed that 80% of current devices are being used in the 2 profiles with the highest levels of clinical compromise. The INTERMACS Coordinators' Council helped to identify gaps in the characterization of hospitalized patients on temporary assist devices and of homebound patients with resting symptoms, which has led to revised definitions of Profile 3 and 4 and the addition of 2 new modifiers, for temporary circulatory support devices in the hospital, and for frequent rehospitalization of patients at home. Conclusions: Patients considered for mechanical circulatory support can now be classified using the 7 profiles plus 3 modifiers developed through INTERMACS. Further understanding these profiles and their impact on outcome should help to better select patients and therapies in the advanced stages of disease.