The role of physical size and strength in sex differences in violence is examined using a sample of ex-offenders, ex-mental patients, and the general population. In incidents not involving weapons, males are more likely than females to engage in attacks and injure their adversaries, and females are more likely to be attacked and injured, primarily because of differences in physical power. In incidents involving weapons, the greater power of males is partially neutralized, and females are more likely than males to injure their adversary. The results show that physical differences between men and women are an important factor in explaining sex differences in violence. Physical power is not the only factor that explains sex differences in producing injury. It appears that males are more effective or skillful in producing injury with bodily force. In incidents involving weapons, in which such skill is less relevant, males are less likely to produce injuries. Footnotes, figure, tables, references