A variety of diseases are associated with the development of unilateral pleural effusion. Although unilateral pleural effusion is common, refractory unilateral pleural effusion is rare. It is important to make an accurate diagnosis using proper diagnostic tools. Thrombocytopenia is one of the rare conditions occurring from various diseases such as severe infection or autoimmune diseases. It can be life-threatening if accurate diagnosis and treatment are delayed and be a clue to accurate diagnosis in differential diagnosis from refractory pleural effusion. Kasabach-Merrit syndrome (KMS) is often accompanied by extensive vascular tumors and characterized by consumptive coagulopathy with profound thrombocytopenia. It is also important to have a high index of suspicion for the diagnosis. We report a case of KMS in a 2-month-old female infant with a vascular tumor on her left intrathoracic cage, who had presented refractory unilateral pleural effusion and thrombocytopenia. Initially, the patient was diagnosed as having complications of severe infection, and a chest tube was inserted for aggressive treatment. However, her unilateral pleural effusion persisted, and thrombocytopenia and hypofibrinogenemia were refractory. Chest imaging revealed an infiltrating large vascular tumor involving the cardiac border, diaphragm, and chest wall. The patient’s unilateral pleural effusion was misidentified as an infectious condition at the initial stage. As a result of the ultrasonography-guided biopsy, it was revealed to be Kaposiform hemangioendothelioma. The patient was cured after treatment for KMS.