Bipolar spectrum disorders (BSDs) are often characterized by cognitive inflexibility and affective extremities, including “extreme” or polarized thoughts and beliefs, which have been shown to predict a more severe course of illness. However, little research has evaluated factors that may be associated with extreme cognitions, such as personality disorders, which are often characterized by extreme, inflexible beliefs and are also associated with poor illness course in BSDs. The present study evaluated associations among BSDs, personality disorder characteristics, and extreme cognitions (polarized responses made on measures of attributional style and dysfunctional attitudes), as well as links between extreme cognitions and the occurrence of mood episodes, among euthymic young adults with BSDs (n = 83) and demographically matched healthy controls (n = 89) followed prospectively for 3 years. The relationship between personality disorder characteristics and negative and positive extreme cognitions was stronger among BSD participants than among healthy controls, even after statistically accounting for general cognitive styles. Furthermore, extreme negative cognitions predicted the prospective onset of major depressive and hypomanic episodes. These results suggest that extreme cognitive styles are most common in individuals with BSDs and personality disorder characteristics, and they provide further evidence that extreme negative cognitions may confer risk for mood dysregulation.