We report here on the social behavior of 30 adult male cynomolgus monkeys, maintained in social groups of five animals each and assigned for 22 months to one of two dietary conditions: a) "luxury"--relatively high fat, high cholesterol (43% calories from fat, 0.34 mg cholesterol/Calorie of diet); or b) "prudent"--relatively low fat, low cholesterol (30% calories from fat, 0.05 mg cholesterol/Calorie of diet). The dietary manipulation resulted in higher total serum cholesterol (TSC) and lower high density lipoprotein cholesterol (HDLC) concentrations in luxury diet animals than in their prudent diet counterparts (p's less than 0.05). Additionally, we monitored the occurrence of 21 behavioral acts frequently exhibited by this species in captivity. Of these behaviors, only contact aggression differed between dietary conditions (p less than 0.03), with prudent diet monkeys initiating more aggression than luxury diet animals. These results are consistent with studies linking relatively low serum cholesterol concentrations to violent or antisocial behavior in psychiatric and criminal populations and could be relevant to understanding the significant increase in violence-related mortality observed among people assigned to cholesterol-lowering treatment in clinical trials.