A 30-year-old woman with severe pathological gambling and cyclothymia presented to our program with no previous history of pharmacologic or psychotherapeutic treatment. Pathological gambling is an impulse -control disorder not otherwise specified (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition) in which comorbidity is common, particularly with substance abuse, obsessive-compulsive disorder and mood disorders. As described in this case, pathological gamblers with bipolar comorbidity may be effectively treated with mood stabilizers such as lithium. After receiving 10 weeks of lithium treatment, the patient showed improvement in both gambling behavior and affective instability. The identification of specific subtypes among patients with pathological gambling may be relevant to the choice of pharmacologic treatment.