Primary prevention trials which have shown that the lowering of serum cholesterol concentrations in middle-aged subjects by diet, drugs, or both leads to a decrease in coronary heart disease have also reported an increase in deaths due to suicide or violence. There has been no adequate explanation for this association. I have reviewed the relevant published work and describe a physiological mechanism that might account for this curious finding. One of the functions of serotonin in the central nervous system is the suppression of harmful behavioural impulses. When mouse brain synaptosomal membrane cholesterol is increased there is a pronounced increase in the number of serotonin receptors. Low membrane cholesterol decreases the number of serotonin receptors. Since membrane cholesterol exchanges freely with cholesterol in the surrounding medium, a lowered serum cholesterol concentration may contribute to a decrease in brain serotonin, with poorer suppression of aggressive behaviour.