Epidemiologic studies link plasma cholesterol reduction to increased mortality rates as a result of suicide, violence, and accidents. Deficient central serotonergic activity is similarly associated with violence and suicidal behavior. We investigated the relationship among dietary and plasma cholesterol, social behavior, and the serotonin system as a possible explanation for these findings. Juvenile cynomolgus monkeys (eight female and nine male) were fed a diet high in fat and either high or low in cholesterol. We then evaluated their behavior over an 8-month period. Plasma lipids and cerebrospinal fluid metabolites of serotonin, norepinephrine, and dopamine were assessed on two occasions, at 4 and 5.5 months after the initiation of behavioral observations. Animals that consumed a low-cholesterol diet were more aggressive, less affiliative, and had lower cerebrospinal fluid concentrations of 5-hydroxyindoleacetic acid than did their high-cholesterol counterparts (p < .05 for each). The association among dietary cholesterol, serotonergic activity, and social behavior was consistent with data from other species and experiments and suggested that dietary lipids can influence brain neurochemistry and behavior; this phenomenon could be relevant to our understanding of the increase in suicide and violence-related death observed in cholesterol-lowering trials.