Six monozygotic (MZ) twin pairs discordant for bipolar disorder were compared with normal MZ twins with magnetic resonance imaging (MRI) on volumes of basal ganglia (BG), amygdala-hippocampus (AH), and cerebral hemisphere. Caudate nuclei were larger in both affected and unaffected bipolar twins than in normal MZ twins. The right hippocampus was smaller in the sick vs. well bipolar twins. The hippocampus was also less asymmetric in the affected bipolar twins than in the well cotwins and the normal MZ twins. These anatomical structures continue to be of interest in bipolar disorder research.