We investigate the prevalence, specificity and possible aetiology of Disinhibited Attachment Disorder (DAD) in adopted children without a history of institutional care. Sixty children adopted from UK out-of-home care (AD; mean age 102 months, 45 % male); 26 clinic-referred children with externalizing disorder (ED; mean age 104 months, 77 % male) but no history of maltreatment or disrupted care; and 55 matched low-risk comparison controls (LR; mean age 108 months, 49 % male) were assessed for DAD using a triangulation of parent, teacher, and research observations. Maltreatment history and child psychiatric symptoms were obtained from parent report and child language development was assessed. DAD was identified in 49 % of AD, 4 % of ED and 6 % of LR children. Seventy-two percent of AD children had suffered maltreatment. DAD was not associated with degree of risk exposure, demographics, or language. A significant association with ADHD did not explain variance in DAD prevalence across groups. DAD was significantly more common in children first admitted to out-of-home care between 7 and 24 months, independent of maltreatment severity, age at adoption and number of care placements. Implications for developmental theory, adoption policy and clinical application are discussed.