A tension hydrothorax is a massive pleural effusion that leads to hemodynamic instability. Here we present a case of tension hydrothorax secondary to poorly differentiated carcinoma. A 74-year-old male smoker presented after a one-week history of dyspnea and unintentional weight loss. Physical exam demonstrated tachycardia, tachypnea, and decreased breath sounds diffusely over the right lung. Imaging revealed a massive pleural effusion causing mass effect on the mediastinum with tension physiology. Chest tube placement revealed an exudative effusion with negative cultures and cytology. Pleural biopsy revealed atypical epithelioid cells consistent with poorly differentiated carcinoma.