In research conducted over the past 20 years, we have observed abnormal trace-metal concentrations, including elevated serum copper and depressed plasma zinc, in blood samples collected from violence-prone individuals. The purpose of the study reported here was to test the validity of our observation that assaultive young males have elevated blood copper/zinc (Cu/Zn) ratios when compared to a control group of young males with no history of assaultive behavior. All male patients between the ages of 3 years and 20 years who made a first visit to the outpatient Pfeiffer Treatment Center in Naperville, Ill., during a two-month period were evaluated. Based on interviews with patients and their families and application of a standardized behavior scale, 135 assaultive young males and 18 controls with no history of assaultive behavior were identified. Blood samples were collected from test subjects and controls and analyzed for serum copper and plasma zinc concentrations by an independent laboratory using atomic absorption methods. The median Cu/Zn ratio for the assaultive subjects was 1.40 compared to 1.02 for controls, a statistically significant difference (t = 5.94; p < 0.01).