The authors conducted a double-blind, placebo-controlled study of the effect of lithium on aggressive behavior. The 66 subjects, who were prisoners in a medium security institution, ranged in age from 16 to 24 years, were physically healthy and nonpsychotic, and had histories of chronic impulsive aggressive behavior. Subjects received lithium or placebo daily for up to 3 months. There was a significant reduction in aggressive behavior in the lithium group as measured by a decrease in infractions involving violence. The authors suggest that lithium can have a clinically useful effect upon impulsive aggressive behavior when this behavior is not associated with psychosis.