Introduction: Inflammatory myofibroblastic tumour (IMT) is a rare intermediate-grade neoplasm in children, most commonly affecting the lung. It frequently mimics infection or malignancy , posing significant diagnostic challenges. Case presentation: A 13-year-old previously healthy boy presented with one month of fever, weight loss, headache, back pain and anorexia. There was no cough or tuberculosis contact history . Examination revealed emaciation, pallor, fever, finger clubbing and reduced left-sided air entry with stony dullness. Inflammatory markers were markedly elevated (CRP 175 mg/L, ESR 150 mm/hr), with leukocytosis and anaemia. CECT thorax demonstrated a left lung mass causing complete lung collapse with mediastinal and bilateral axillary lymphadenopathy , raising suspicion of lymphoma. Axillary lymph node biopsy and bone marrow examination were reactive. Initial mediastinal biopsy showed chronic inflammation. Repeat tru-cut biopsy revealed spindle cell proliferation in an inflammatory background. Immunohistochemistry demonstrated strong diffuse cytoplasmic ALK positivity and SMA positivity , consistent with ALKrearranged inflammatory myofibroblastic tumour. Conclusions: IMT should be considered in children presenting with constitutional symptomsand radiological features suggestive of malignancy . ALK immunopositivity is critical for diagnosis and has therapeutic implications. Early multidisciplinary evaluation and adequate tissue sampling are essential to avoid diagnostic delay and guide appropriate management.