A right-handed 42-year-old man with bipolar II disorder according to the Research Diagnostic Criteria was tested during two depressive phases, a hypomanic phase, and after a series of left ECT treatments. While drug free he was given a set of tasks sensitive to asymmetry in hemispheric activation. The results indicated hyperactivation of the right hemisphere during the hypomanic episode and of the left hemisphere during the depressive episodes. After left ECT, the patient evidenced greater symmetry. The authors suggest that this case is an instance of reversed lateralization in the regulation of positive and negative mood states.