There is increasing evidence for structural differences in the brains of patients with affective disorders. Recent magnetic resonance imaging (MRI) studies have reported focal signal hyperintensities in the deep white matter of bipolar patients. These previous reports had focused on adult patients with prior episodes of illness. In this case report, the authors discuss a young adolescent patient during her first episode of mania and the finding of subcortical focal signal hyperintensities on brain MRI. The etiology, pathophysiology, and clinical correlates of these lesions will be reviewed.