The current study sought to determine the prevalence, severity, and location of white matter signal hyperintensities (WMH) on brain magnetic resonance imaging assessments of children and adolescents with psychiatric disorders. Seventy-one percent (N = 934) of children admitted to the McLean Hospital Child and Adolescent inpatient treatment unit were evaluated with the Diagnostic Interview Schedule for Children (DISC) within 7 days of admission during the period 1988 to 1993 (total, 1, 308 admissions). Four hundred eight of these subjects (43.7%) were referred for brain magnetic resonance imaging (MRI) scans and became our study subjects (mean age, 12.4 years [SD = 2.7]; male/female, 230/178). Study subjects were grouped according to a hierarchical diagnostic system as follows: schizophrenia (n = 42), bipolar disorder (n = 56), unipolar depression (n = 94), conduct disorder/attention deficit disorder (n = 103), and other neurotic disorders (n = 30). Subjects without any level 2 diagnosis on DISC (n = 83) constituted the comparison group. Bipolar disorder, unipolar depression, and conduct disorder/attention deficit disorder groups were significantly more likely to have severe levels of WMH than the comparison group (prevalence rates: 17.9%, 13.8%, 13.6% v 1.2%). In addition, the bipolar disorder group was significantly more likely to have severe levels of WMH than the schizophrenia group (prevalence rates: 17.9% v 2.4%). The frontal lobes were the predominant locations of WMH in the bipolar disorder and unipolar depression groups (76.9% and 60.0%, respectively) and also the most frequent location for the conduct disorder/attention deficit disorder group (35.7%). The current study reports an increased prevalence and severity of WMH in children with bipolar disorder, unipolar depression, and conduct disorder/attention deficit disorder relative to the comparison group and in children with bipolar disorder compared to those with schizophrenia. The development of brain WMH, especially in the frontal lobes, may play a role in the pathophysiology of affective disorders in children and adolescents.