On an afternoon in early April 2010, the outpatient clinic for mood disorders was busy and tense as usual. Suddenly, the door of the clinic was hastily pushed open and in walked a middle-aged man with both eyebrows locked and agitated. “Doctor, please help me, it’s too painful, I can’t stand it anymore!” Let’s call this visitor “Old Li”. Old Li was very satisfied with his life, with a successful career and a happy family. But four years ago, all that changed. He suddenly fell into an inexplicable nervousness and anxiety, feeling depressed every day, spending days like years, and there seemed to be no more things to be happy about in his life, even his beloved career, which made him tired and even wanted to leave. Old Li was very painful, saw many doctors, said he was “depression”, has been taking antidepressants, but the condition has been repeated, never completely relieved, and even more and more serious. Every year, there are one or two months, Lao Li will feel the passion back to himself, he feels energetic, quick thinking, a night can complete dozens of pages of work reports. But the good times are always so short, and it doesn’t take long for Lao Li to fall back into a state of pessimism and despair, and the depression lasts longer and longer. Although the dose of antidepressants has been increased, his condition has not improved. After four years of being tortured by the disease, Lao Li almost collapsed and finally came to our specialized outpatient clinic on the advice of his local doctor. Why was Lao Li’s disease so difficult to treat? The crux of the problem is that Lao Li is not a simple “depressed” patient. Old Li’s depressive symptoms were so prominent that we overlooked the fact that he actually had the manifestation of hypomania, and that the passionate “good times” every year were actually the manifestation of hypomania. To be precise, his diagnosis should be “bipolar disorder – type II”. Bipolar disorder-Type II refers to a patient who has a typical depressive episode and a hypomanic episode. Hypomania is clinically more common than true manic episodes, but is often overlooked and missed. Light mania often manifests as happy mood, quick thinking, high energy, increased social activities, and reduced need for sleep, etc. Sometimes it is difficult to distinguish from normal people “in good condition”. Moreover, the feeling of “hypomania” is very comfortable, and the patient will not actively seek medical help for this reason. Therefore, hypomania is often difficult to be detected and becomes an “invisible killer” that endangers the health of patients with bipolar disorder – type II. Timely and accurate identification of hypomania is the key to the diagnosis and treatment of bipolar disorder-II. For patients with bipolar disorder-II, the correct treatment strategy is to use mood stabilizers as the mainstay, supplemented by new antipsychotics, so as to achieve the purpose of mood stabilization and symptom relief. If we use antidepressants alone to treat patients with mild manic symptoms, the result will be like Old Li, where the good times are short and the suffering is long and prolonged. For the sake of stability and long-term health, let’s work together to find out the “invisible killer” of hypomania.