This study compared the adult mental health consequences of childhood maltreatment and bullying from peers in participants in two longitudinal studies, one in the UK and the other in the USA. The findings showed that being bullied by peers in childhood generally had worse long-term adverse effects on young adults' mental health. The results consistently showed an increased risk of young adult mental health problems such as anxiety, depression, self-harm or suicidality in children who were bullied by peers, whether or not they had a history of maltreatment by adults. When being bullied was directly compared with maltreatment in childhood, being bullied by peers had more adverse eﬀects on early or young adult overall mental health. Maltreatment mainly had adverse eﬀects on mental health problems when the children had also been bullied by peers.