Localized interlobar effusion in congestive heart failure (known as phantom tumor or vanishing tumor of the lung) is an uncommon entity. We report a case of a 61-year-old man who presented to the Emergency Departament with a two week history of dyspnoea, palpitations, dry cough and intermitent anterior chest pain. A posteroanterior chest radiography showed a nodular mass in the medium third of the right hemithorax suggestive of a pulmonary tumor. With this supposition of diagnosis, the patient was admitted to the Pneumology Departament for further investigations. Left ventricular systolic dysfunction was identified on the echocardiographic examination, in the presence of atrial flutter with 2 to 1 block. Lateral chest X-ray confirmed the presence of a pleural effusion with complete regression of "the lung tumor" after ten days of congestive heart failure treatment, avoiding other expensive and unnecessary investigations.