(Can J Psychiatry 2006:51:6-8) The concept of a broad and inclusive bipolar spectrum disorders appear to be gaining momentum in the psychiatric literature, although cautionary comments have previously been made (for example, by Baldessarini; 1). The possibility that a large proportion of people diagnosed with depression actually have BD is an important clinical consideration and a natural corollary of this trend. I will not focus specifically on the unipolar-bipolar distinction, however, since the general trend toward broadening the diagnostic boundaries of BD raises similar issues for many other conditions (such as impulse control disorders and Axis II pathology). Rather, I will direct my comments toward the broad, ongoing debate about the bipolar spectrum concept. My intention is not to argue that existing diagnostic conventions are perfect or that a bipolar spectrum does not exist but to draw attention to certain issues that have not received adequate emphasis in the literature. These issues include setting appropriate diagnostic thresholds, the inevitable tension in psychiatry between empirically oriented and theory-driven concepts, and debate over the relative merits of categorical and dimensional measurement. The arguments put forward in favour of a broad bipolar spectrum are protean. In 2000, Akiskal and colleagues provided a detailed review (2). Some of the more notable arguments are that hypomanic episodes may last less than the 4-day threshold specified in the DSM-IV, that certain subsets of subjects without a DSM-IV BD have a higher-than-expected proportion of relatives with BDs, that mixed states occur frequently and are underrecognized clinically, that patients with manic or hypomanic episodes induced by antidepressants often later manifest bipolar states, and that some subjects who do not meet DSM-IV criteria for a BD respond to mood stabilizers. There is formally accepted definition of what is meant by the term bipolar spectrum. The published works of various authors have adopted quite different emphases. To some authors, the idea of a spectrum implies a spectrum of symptom severity, resulting in the argument that a larger or milder set of bipolar symptoms should be accepted as fulfilling diagnostic criteria for hypomanic or mixed episodes. For some, the emphasis has been more deeply conceptual, typically targeting what is seen as an artificial distinction in the DSM-IV between unipolar and bipolar disorders. In this latter instance, the spectrum concept can be understood not simply as the broadening of a diagnostic category but as a more basic reappraisal, perhaps one that favours dimensional measurement. The DSM-IV includes a kind of bipolar spectrum: BD I and BD II, as well as cyclothymic disorder. These are classified together with other mood disorders. Mixed states are treated much like manic states in the 6 DSM-IV coding categories for BD I. Further, the categories are fluid: a manic episode can move an afflicted individual from one category (such as major depressive disorder, recurrent) to another (such as BD I, most recent episode manic). However, the movement occurs as a result of signs and symptoms identified according to explicit diagnostic criteria. The classification operates within a fruitful empirical tradition established by the DSM-III, which attempts to be neutral with respect to etiologic theory. Some authors supporting the concept of a broad bipolar spectrum have emphasized that some patients diagnosed originally as suffering from unipolar depression later develop manic, hypomanic, or mixed episodes. Often, this observation is offered as if it provided self-evident confirmation of problems with the existing nosology-but it is rarely acknowledged that this is how the criteria are designed to work. The DSM-IV makes no assumption that all individuals described as having the same disorder are alike in all important ways (3, Introduction, ρ xxxi). Returning to a diagnostic approach that depends on theoretical assumptions about what bipolarity theoretically means (including assumptions about underlying pathophysiology) can jeopardize the advantages that an empirical approach offers. …