There are long-standing viewpoints that impulsive and violent behavior may stem from brain dysfunction or damage secondary to head injury, disease, or toxic chemical substances. This research has aimed to examine the relationship between potentially toxic metals and aberrant behavior, especially violent activity, through the nonintrusive technique of hair analysis for trace elements. In an initial study, phase I, it was not possible to replicate findings of others who reported high levels of lead, cadmium, and copper in violent offenders. However, high levels of manganese were found in prison versus control groups. In phase II, the possibility of artifactual results arising from prison cooking utensils was controlled for by sampling early after incarceration. Phase III was included to substantiate the initial post hoc findings in an additional jail population. In both latter phases, significantly elevated manganese levels were found in the hair of violent versus nonviolent subjects (P less than .0001). A review of the effects of manganese at deficient and toxic levels does not provide a simple answer as to why manganese levels are elevated in the hair of individuals who have been incarcerated for violent behavior. Our study does not implicate the prison environment or soaps and shampoos used in California prisons. Other factors, such as alcohol, dietary, or psychosocial factors, might influence manganese levels in hair, or any of these factors might function in combination with mild manganese toxicity to contribute to aberrant behavior.