A 17-year-old male with a previous history of T-cell nonHodgkin lymphoma was noted on follow-up to have a white cell count of 79·1 9 10/l and an eosinophil count of 8·8 9 10/l. Peripheral blood film shown (top left). Immunophenotyping confirmed precursor B-cell acute lymphoblastic leukaemia (ALL). Cytogenetic analysis demonstrated t(5;14)(q31;q32), t(7;12)(p15;q13), trisomy 19 and monosomy 20 (top right). Fluorescent in situ hybridization for FIP1L1-PDGFRA was negative. During induction chemotherapy (dexamethasone, daunorubicin, vincristine and asparaginase), he developed chest pain. Cardiac magnetic resonance imaging (CMR) was performed (bottom). This showed a small pericardial effusion, normal biventricular size and function, a small left ventricular apical thrombus, myocardial oedema and extensive subendocardial late enhancement, classical for eosinophilic infiltration. A thrombophilia screen demonstrated heterozygosity for F2 G20210A. Despite anticoagulation with low molecular weight heparin (LMWH), he developed lower limb deep vein thromboses, pulmonary emboli and a left cephalic vein thrombosis. He subsequently died of progressive disease after a myeloablative allograft. Eosinophilic ALL with t(5;14)(q31;q32) is rare. Loeffler myocarditis, diagnosed on CMR, showed three typical features of myocardial involvement (oedema, thrombus and late enhancement). We suggest this noninvasive procedure as an alternative to tissue diagnosis of Loeffler endocarditis. Extensive thromboembolism has been reported in B-ALL with t(5;14) translocation and we therefore suggest consideration of thromboprophylaxis with LMWH.