Growing evidence for the spectrum concept of most mental disorders, particularly mood disorders, has challenged the arbitrary distinctions inherent in the contemporary categorical diagnostic classification system. Detection of manic symptoms in the context of episodes of depression is particularly important because of the implications for differential treatment of bipolar vs unipolar depression. The purpose of this study is to characterize the magnitude and clinical correlates of subthreshold manic syndromes or symptoms among people with major depressive disorder (MDD) compared to those without a history of manic symptoms. We defined two subthreshold manifestations—manic syndrome or symptoms—that did not include a criterion for duration. In the context of MDD, we found that the clinical correlates of those with the subthreshold manic syndrome were more similar to those with bipolar-II disorder than to MDD alone, whereas those with manic symptoms only were intermediate between those with subthreshold manic syndrome and MDD alone. These results confirm the spectrum concept of mania and suggest that a manic syndrome should be considered when evaluating people with MDD.