First hospital admission of 20 years old male known case of sickle thalassemia transferred from another hospital with impression of acute leukemia. He was well until one week PTA when he developed R-knee pain and gradually experienced generalized bone pain. Referred to hospital and was admitted. Results of laboratory test at the time of admission were:WBC=7100, BUN=27, BS=142, HB=12.1, CREA=0.6, Na=141, PLT=72000, Ca++=9.1, K+=4.4, MCV=67, PO4=4.5, PT=19.5, INR=2.3 He gradually developed dizziness, fever and episodes of epistaxis and pallor.Results of laboratory tests 4 days after admission were: WBC=34100, LDH=3353, CPK=142, Hb=4.5, Plt=28000 Abdominal sonography revealed gallstone and huge splenomegaly.BMA: suggestive of acute leukemia.He transfused 4 units of pached RBC and 7 units of platelets.Dexamethazone: 8 mg BID, Ciprofloxasine: 200mg IV q 12h, Ceftriaxone: 1mg IV q 12h, Allopurinol: 100mg q 8h started for the patient. At 6th hospital day he transferred to our hospital with impression of acute leukemia.FH: father has sickle cell trait and mother has thalassemia minor.On physical examination patient was pale, Bp: 90/70, PR=120, Temp=38.5, RR=20 Positive finding at time of admission were: pallor, mild jaundice, huge splenomegally no petechia or purpura.Vancomycin, imipenem started for patient. Results of BMA and BMBX showed extensive necrosis of BM and many degenerated cells. Flowcytometry was: normal WBC=31100, PARVO VIRUS B19 IGM= NEGATIVE, HB=9.8, COOMB'S TEST= NEGATIVE, PLT=57000, G6PD= NEGATIVE, RETIC=1%, LDH=5000 CT of abdomen showed huge splenomegally and multiple S.O.L in spleen suggestive of infarction.Splenectomy and cholecystectomy was done at 10th day of hospital admission.We will discuss this interesting case and differential diagnosis for him. He suffered from splenic sequestration crisis a rare type of crisis in sickle cell disease and sickle thalassemia.