Bipolar type II depression (BD II) is a less severe type of bipolar disorder. BD II is characterized by one or more depressive episodes and at least one hypomanic episode. During hypomanic episodes, people experience especially energetic or anxious moods, and their thoughts are more sporadic than usual, but they do not experience the severity of mania. Symptoms of BD II are known to impair daily functioning as well as cause distress and even suicide. Antidepressant medication alone is not recommended for people with bipolar disorder because manic symptoms usually worsen. It is unknown whether the same recommendation should apply to people with BD II. Sertraline is a selective serotonin reuptake inhibitor (SSRI) antidepressant that increases levels of serotonin, helping the brain to maintain mental stability. It is often used to treat depression, panic attacks, and other disorders. Lithium is a mood stabilizing medication that decreases abnormal brain activity and is used to treat and prevent recurring episodes of mania in people with bipolar disorders. This study will evaluate the effectiveness of lithium alone, sertraline alone, and lithium with sertraline to treat symptoms of BD II.