Retroperitoneal leiomyosarcoma arising in a spermatic blood vessel is described. In addition to typical leiomyosarcoma, areas of atypical histiocyte‐like cells and storiform pattern bearing a close resemblance to malignant fibrous histiocytoma, were seen. Ultrastructurally, cells with features of smooth muscle, partially differentiated cells with marginal densities and basal lamina and histiocyte‐like giant cells were present. Phagocytosis of inflammatory cells by tumour cells with marginal densities and an investment of basal lamina was observed. It is suggested that the precursor cell in this lesion was a primitive mesenchymal cell showing varying degrees of smooth muscle differentiation and giant cell format