The study found smaller amygdala volumes in children and adolescents with bipolar disorder compared with the control children and adolescents; however, amygdala volumes in bipolar adults were not significantly different from the control adults. There are several possible explanations for these differences in amygdala volumes of bipolar youths and adults. One possible explanation is that the smaller amygdala volumes in the bipolar youths do not persist into adulthood because of a compensatory reaction of the body through such mechanisms as increased blood flow and hypertrophy. Such changes of amygdala physiology, particularly in the context of genetic predisposition, could lead to abnormal regulation of mood and behavior in those with symptoms of bipolar disorder. Other aspects of disease progression, including the influences of medication exposure and degree of substance abuse, may also account for the inconsistency of amygdala volumes across age groups. Also, differences between amygdala volumes in adults and child samples of bipolar patients may be due to distinctions in the underlying path physiology of early-onset bipolar disorder compared with late-onset bipolar disorder. Future studies should focus on age-specific biomarkers of illness and treatment response. The literature search used the National Institutes of Health’s PubMed in order to identify published peer-reviewed neuroimaging studies of amygdala size in children, adolescents, and adults with bipolar disorder. Eleven studies met inclusion and exclusion criteria. 4 tables, 2 figures, and 41 references