We reviewed available evidence regarding a possible relationship between impulse-control disorders (ICDs) and bipolar disorder. Studies examining the phenomenology, course, comorbidity, family history, biology, and treatment response of ICDs were compared with similar studies of bipolar disorder. Although no studies directly compare a cohort of ICD patients with a cohort of mood disorder patients, available data suggest that ICDs and bipolar disorder share a number of features: (1) phenomenologic similarities, including harmful, dangerous, or pleasurable behaviors, impulsivity, and similar affective symptoms and dysregulation; (2) onset in adolescence or early adulthood and episodic and/or chronic course; (3) high comorbidity with one another and similar comorbidity with other psychiatric disorders; (4) elevated familial rates of mood disorder; (5) possible abnormalities in central serotonergic and noradrenergic neurotransmission; and (6) response to mood stabilizers and antidepressants. However, ICDs and bipolar disorder differ in important respects. In particular, some ICDs may be more closely related to obsessive-compulsive disorder (OCD) than is bipolar disorder. Although the similarities between ICDs and bipolar disorder may be coincidental, they suggest that the two conditions may be related and thus may share at least one common pathophysiologic abnormality. To explain this possible relationship, we hypothesize that impulsivity and bipolarity (or mania) are related, that compulsivity and unipolarity (or depression) are similarly related, and that each state may represent opposing poles of related, or even a single, psychological dimension.