It has been suggested that psychological interventions for personality disorders should focus on improving adaptive expression of the functional needs expressed through problematic behaviors such as aggression. The measurement of function is a necessary condition for devising a function-based treatment approach. Two studies that employ a method for assessing the functions of aggressive behavior in personality disordered patients are described by: (a) a retrospective analysis, through file review of inpatient aggressive behaviors and of the index offenses of patients admitted for treatment, and (b) an analysis of inpatient aggressive incidents where staff and patients were interviewed to elicit functions. Results showed that most aggression had the function of expressing anger, although other functions were also evident. The profile of functions observed in the inpatient studies was somewhat dissimilar to that observed in the studies of predominantly psychotic patients. Therapeutic implications of these findings are discussed. (Contains 1 table.)