We have recently reported that memantine has a clinically relevant antimanic and long-lasting mood-stabilizing effect in treatment-resistant bipolar disorders, both as augmenting agent and as monotherapy. We have also observed an acute antimanic and sustained mood-stabilizing effect in a small number of patients with bipolar I disorder who had had minimal previous pharmacotherapy. In this article, we report the case of a young woman suffering from bipolar II disorder with associated fibromyalgia, in whom memantine showed an acute antimanic and a long-term prophylactic effect on both bipolar disorder as well as the associated fibromyalgia syndrome.