Sex is clearly important in unipolar mood disorder with compelling evidence that depression is approximately twice as common in women than in men. In the case of bipolar disorder, however, it is widely perceived that the reported equal rate of illness in men and women reflects no important gender distinctions. In this paper we review the literature on gender differences in bipolar illness and attempt to summarize what is known and what requires further study. Despite the uncertainties that remain some conclusions can be drawn. Most studies, but not all, report an almost equal gender ratio in the prevalence of bipolar disorder but the majority of studies do report an increased risk in women of bipolar II/hypomania, rapid cycling and mixed episodes. Important gender distinctions are also found in patterns of co-morbidity. No consistent gender differences have been found in a number of variables including rates of depressive episodes, age and polarity of onset, symptoms, severity of the illness, response to treatment and suicidal behaviour. Unsurprisingly, however, perhaps the major distinction between men and women with bipolar disorder is the impact that reproductive life events, particularly childbirth, have on women with this diagnosis.