Aim: To present the case of a pediatric patient with superior mediastinal syndrome (SMS) in whom a preliminary diagnosis of T-cell non-Hodgkin lymphoma (T-NHL) was established by cytological analysis of pleural and pericardial effusion and due to the life-threatening condition, chemotherapy was initiated with an excellent response. Case report: A 15-year-old girl was examined in the emergency pediatric department due to unspecified loss of consciousness. Five days before the admission, she was complaining of tinnitus, dizziness, night sweats, diffuse abdominal pain and vomiting, and dry cough on the day of the admission. Her clinical condition worsened rapidly with cyanosis and hypoxia, and she was admitted to the Intensive Care Unit. Imaging studies demonstrated a mediastinal mass, and large pericardial and pleural effusion. Along with intensive supportive measures, pericardiocentesis and pleural drainage were performed. Cytological examination of fluids established a preliminary diagnosis of T-NHL, and chemotherapy according to EURO-LB 02 protocol was started without histological confirmation, with excellent response. Five years after the end of the treatment, the patient is in complete remission. Conclusion: SMS is an emergency in pediatric oncology. The rapid development of typical SMS signs and symptoms in a child should arouse clinical suspicion of a mediastinal tumor. In life-threatening cases, along with intensive care treatment, it is necessary to start anticancer therapy based on a preliminary diagnosis without histological confirmation. Early identification and adequate treatment of SMS are crucial for a favorable outcome.