Abstract: 
A 48-year-old male diagnosed with Acute promyelocytic leukemia (APL) started on all-trans retinoic acid and arsenic trioxide, developed typical symptoms of differentiation syndrome, and improved dramatically on steroids. Hence, any APL patient started on chemotherapy, needs to be monitored closely for developing differentiation syndrome and to start steroid upon suspicion.
Case presentation:
A 48-year-old male diagnosed with APL started on all trans-retinoic acid (ATRA) and arsenic trioxide(ATO) as per protocol (1), on day 2 he started to have fever, dyspnea, hemoptysis. Chest X-ray (CXR) showed bilateral infiltration (Figure A) and patient was started on dexamethasone, based on suspicion of differentiation syndrome. Patient improved significantly within 24 hrs, and follow-up CXR showed resolution of infiltrates (Figure B). Diagnosis of differentiation syndrome requires 3 or more of clinical features: fever> 38 c , weight gain >5 kg , hypotension, dyspnea, radiographic opacities, pleural or pericardial effusion & Acute renal failure (2). Treatment is dexamethasone 10 mg twice daily initially, and then to adjust the steroid dose based on patient clinical status. Differentiation syndrome is a fatal complication therefore there should be high index of suspicion in APL patients who deteriorates after starting chemotherapy and to start treatment empirically.