Introduction: Mediastinal tumours in children often masquerade as common respiratory infections. A failure to respond to standard therapy should prompt early diagnostic imaging to identify alternative underlying pathologies. Case presentation: A 13-year-6-month-old girl with bronchial asthma presented with a threeweeks history of generalised body aches and multiple somatic complaints followed by two days of fever. She reported left-sided neck and shoulder pain, a non-specific headache, a three-months non-productive moist cough, and one-week of pleuritic chest pain. Clinical examination revealed respiratory distress, left side stony dullness with reduced air entry and tracheal deviation to the right. Chest radiography demonstrated a large left pleural effusion. Blood tests showed neutrophil leucocytosis, CRP 15 mg/L and ESR 62 mm/h. Despite intravenous antibiotics her respiratory distress worsened. Ultrasound-guided pleural aspiration drained 400 mL straw-coloured fluid. Pleural fluid analysis showed >10,000 pus cells, Lactate Dehydrogenase 1,368 U/L and protein 34 g/L. She was therefore managed as complicated pneumonia with empyema. However, contrast-enhanced CT on day 7 demonstrated a large mediastinal mass (7 .5 × 10 × 15 cm) extending from C7 to T5 and reaching the lower thyroid border. Biopsy confirmed lymphoblastic lymphoma, and she was transferred to Apeksha Hospital for definitive oncological management. Conclusions: Mediastinal malignancy must be considered in children with respiratory symptoms refractory to conventional treatment. Early cross-sectional imaging can prevent diagnostic delay and facilitate timely specialist referral.