Secondary sea-blue histiocytosis occurs more frequently than the primary form and occurs consequent to a wide range of metabolic and haematologic disorders including thalassaemia. We report an 18-year-old Chinese boy with transfusion-dependent HbE-beta thalassaemia who complained of pain and swelling at the left iliac crest region for 2 months duration. Physical examination revealed pallor with hepatosplenomegaly. Local examination revealed a huge swelling 12 cm × 12 cm in diameter, firm in consistency and tender. Histopathological examination of the mass revealed an osteosarcoma. His bone marrow aspirate showed numerous sea-blue histiocytes, the cytoplasm of which was closely packed with fine granules that stained blue with May-Grunwald-Giemsa. The nuclei were centrally located in some cells and displaced towards the periphery in other cells. There was no malignant cell infiltration in the marrow. The case is reported due to the co-incidental dual pathology in our patient (HbE-beta thalassaemia and osteosarcoma) and the unusual bone marrow finding of numerous sea-blue histiocytes.