Delayed diagnosis or misdiagnosis can prolong the suffering of patients with bipolar disorder. Accurate early diagnosis is sometimes difficult, however, particularly because patients often present in the depressive phase, which can easily be mistaken for unipolar depression. Unfortunately, therapy appropriate for unipolar depression can increase the risk of manic switch or cycle acceleration in bipolar disorder, especially in those with a family history of bipolarity and suicide, although some antidepressants may be useful in some bipolar patients. In addition, most currently available mood stabilizers, though effective in managing mania, do not effectively resolve depression. In contrast, lamotrigine has shown activity in bipolar depression and has a very low risk of manic switch. Bipolar depression, compared with unipolar depression, is more likely to be associated with hypersomnia, motor retardation, mood lability, early onset, and a family history of bipolar disorder. Awareness of these distinctions can greatly improve diagnosis of bipolar disorder and provide an opportunity for effective therapeutic intervention.