This study examined the possible association between prolactin (PRL) system genes and callous-unemotional (CU) traits in childhood-onset aggression. Two markers for the PRL peptide gene and three markers for the prolactin receptor (PRLR) gene were genotyped. The participants were assessed on the CU subscale using five items from the Antisocial Process Screening Device. Genotype analysis showed nominally significant results with PRLR_rs187490 (uncorrected P=0.01), with the GG genotype associated with higher CU scores. This is the first paper to evaluate the relationship of PRL system genes with CU traits in childhood-onset aggression.