SUMMARY A rare condition of massive intrathoracic extramedullary haematopoiesis causing spinal cord compression is presented in a 17-year-old Saudi male. Both clinical and laboratory findings, together with the computed tomographic evaluation of the lesion are described. Partial surgical resection as an alternative to radiotherapy has resulted in a dramatic response. dull aching in nature made worse by movement, exertion and cough and relieved by rest. The pain did not radiate to the chest wall or to the lower limbs.He described the pain as mild at first but later it became more severe. Four weeks prior to his hospital admission, he complained of progressive weakness of both lower limbs and finally was unable to walk or support his weight. There was no urinary or bowel disturbance. The patient had no significant past medical history, and no family history of a similar complaint was elicited.Physical examination revealed a thin, pale and ill looking young man who was less physically developed than his stated age. In the upper limbs the humeri looked short in relation to his forearms. The chest wall showed an obvious pigeon-shaped deformity and swelling of all costochondral junctions. The spleen was palpable two fingers breath below the costal margin but the liver was not enlarged. The positive findings on CNS examination were confined to his lower limbs. He had a spastic paraparesis with grade 4 diffuse and bilateral muscle weakness. Hyperreflexia and extensor plantar responses with a sensory level corresponding to D7 were elicited. Laboratory investigations gave the following results: haemoglobin 10 gmJdl, HCT 36%, MCV 62/,um3, MCH 20/pg, MCHC 30%, and normal WBC and differential. The blood film showed hypochromia, microcytosis, polychromasia, target cells, poikilocytosis, anisocytosis, and nucleated erthrocytes. Sickle test was negative. Serum iron was normal. His chest radiograph ( fig la) showed a paraspinal well demarcated mass at the level of T4 on the right side and a similar mass at T10 level. Myelogram demonstrated a total block at T7/T8 due to an extradural posteriorly placed mass. The extent of the mass was confirmed by a CT scan, (fig ib); it was found to extend from T4 to T8. Cellulose acetate haemoglobin electrophoresis in alkaline medium demonstrated haemoglobin F and haemoglobin A2 without haemoglobin A. By the Sniger's alkali denaturation, haemoglobin F was 72%. 780