Man's aggression permeates everyone's life. Although aggression is discussed increasingly, few studies have investigated it in unselected populations. The majority of studies of human aggression have subjects selected because of their violent behavior. Marten et al. 1 called attention to this fact after a literature survey. In their recent report on belligerence, they found a 74% incidence of belligerent thoughts and behavior in their population. The subjects were largely medically indigent psychiatric patients seen in the emergency room of an urban community mental health center. Males were more belligerent than females, young patients more than old, and no direct or inverse relation could be found between belligerent and self-destructive behavior. Gulevich and Bourne 2 concluded from several investigations that discharged psychiatric patients do not show a greater risk of behaving in a criminal or violent way than the general population. Rappeport, 3,4 however, revealed a higher incidence of robbery in discharged male psychiatric patients, and of aggravated assault in discharged female psychiatric patients. This study investigates specific manifestations of aggression among hospitalized psychiatric patients by using a broad definition: the entire spectrum of assertive, intrusive, and attacking behaviors directed at the self or others. 5 Patients were asked a range of questions of gradually increasing intensity about their aggression toward others and toward themselves. If their responses to a question were positive, they were categorized into three levels of expression: thoughts and feelings, verbalization, and physical action. The subjects were not selected because they were violent in some way, but because they had some psychiatric problem or illness that caused them to be hospitalized.