The extent of symptomatology related to attention-deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD) was examined in a statewide sample of adopted youth, aged 4-18 years (n = 808). The use of normed questionnaires in a nonclinical sample decreased biases associated with past research on adopted children. According to parental report, a striking number of the youth qualified as manifesting significant symptom levels of externalizing behavior problems: 21% met symptom cutoffs for ADHD (with or without ODD) and 20% met criteria for ODD (with or without ADHD), for a combined total of 29% of the sample. A number of parent-reported, preadoptive risk factors distinguished these groups from one another and from the nonexternalizing youth. The clearest associated factors included histories of preadoption abuse/neglect, later age of adoption, prenatal drug exposure, and placement in multiple foster homes prior to adoption. We discuss implications regarding both etiology and current controversies surrounding the disproportionate levels of behavioral difficulties in adopted youth.