A primary malignant fibrous histiocytoma of the spleen was studied by light and electron microscopy and is believed to be the first reported. The neoplasm was predominantly fibroblastic, with a characteristic storiform pattern, and included histiocyte-like cells, giant and foam cells confirmed by ultrastructural studies. Additionally, undifferentiated cells, intermediate cells and myofibroblasts were seen. The differential diagnosis from other sarcomas and the histogenesis of this tumour are discussed.