Teratomas are tumors originating from the three primary germ layers, most commonly located within gonads or in the sacral region. Chest locations are rare. Mediastinal teratoma showing no tumor-specific symptoms may be treated as exudative pneumonia. The goal of the article was to present a case encountered in our practice as a showcase of possible diagnostic problems and errors. Despite a thorough medical examination with additional exams (ultrasound scans of pleural cavities, chest X-ray and laboratory analyses), the diagnosis of a thoracic tumor was made only after chest computed tomography scan was performed following ineffective attempts at antibiotic therapy and pleural drainage. Following the diagnosis of mediastinal tumor, the patient was subjected to surgery. A giant teratoma (confirmed in histopathological examination) was removed upon left-sided thoracotomy. Following the procedure, lung expansion and patient recovery were observed. Computed tomography of the chest should be performed routinely upon encountering difficulties in the treatment of exudative pneumonia in children. In every case of pneumonia with pleural effusion in children, inflammatory mask of mediastinal tumors should be ruled out.