A 53-year-old man was admitted to our hospital for treatment of fever and chest pain. Chest computed tomography showed an anterior mediastinal cystic tumor 39×57 mm in size surrounded by fat with edema and left pleural effusion. After one course of antibiotic administration, the edema of fat surrounding the tumor disappeared and the patient underwent scheduled tumor resection with thymectomy through a median sternotomy. Postoperative pathological examination revealed a thymoma of type AB according to the World Health Organization (WHO) classification associated with hemorrhage and necrosis. Cases of thymoma with hemorrhage or necrosis may lead to atypical presentations such as fever, acute chest pain, pleural effusion, and spontaneous regression. Clinicians should be aware of these unusual presentations of chest pain and fever due to thymoma, and consider the possibility of a differential diagnosis of an anterior mediastinal tumor.