Maltreatment places children at risk for psychiatric morbidity, especially conduct problems. However, not all maltreated children develop conduct problems. We tested whether the effect of physical maltreatment on risk for conduct problems was strongest among those who were at high genetic risk for these problems using data from the E-risk Study, a representative cohort of 1,116 5-year-old British twin pairs and their families. Children's conduct problems were ascertained via parent and teacher interviews. Physical maltreatment was ascertained via parent report. Children's genetic risk for conduct problems was estimated as a function of their co-twin's conduct disorder status and the pair's zygosity. The effect of maltreatment on risk for conduct problems was strongest among those at high genetic risk. The experience of maltreatment was associated with an increase of 2% in the probability of a conduct disorder diagnosis among children at low genetic risk for conduct disorder but an increase of 24% among children at high genetic risk. Prediction of behavioral pathology can attain greater accuracy if both pathogenic environments and genetic risk are ascertained. Certain genotypes may promote resistance to trauma. Physically maltreated children whose first-degree relatives engage in antisocial behavior warrant priority for therapeutic intervention.