In 6 XYY patients suffering from aggressiveness and admitted by order of law into a security setting for acts of violence, the estimation of CSF amine metabolites before and after a probenecid test revealed a clear decrease in the turnover of central serotonin (5-HT) while that of dopamine (DA) was unchanged. The treatment with the 5-HT precursor, L-5-hydroxytryptophan (L-5-HTP), in 5 of the XYY patients resulted in a clinical status equivalent to that observed when the patients were previously treated by conventional neuroleptics.