Results : The authors reviewed the evolution of the concept of mixed states and examined the symptom structure of mixed states studied as predominantly manic, predominantly depressive, and across both manic and depressive episodes, showing essentially parallel structures of mixed states based on manic or depressive episodes. The authors analyzed the relationships between mixed states and a severely recurrent course of illness in bipolar disorder, with early onset and increased co-occurring anxiety, stress, and substance-related disorder, and they used this information to derive proposed diagnostic criteria for research or clinical use.