Spindle cell and pleomorphic lipoma are rare benign tumors of adipose tissue. Their histogenesis is poorly understood and pleomorphic lipoma has not previously been studied ultrastructurally. This study describes the morphology and ultrastructure of 6 cases of spindle cell and pleomorphic lipoma. Both spindle cell and pleomorphic lipomas showed similar ultrastructural features with an interrupted basal lamina, occasional non-membrane-bound lipid vacuoles and abundant rER. In all the cases mast cells were conspicuous and were intimately related to both spindle and pleomorphic cells. It is proposed that both the spindle and pleomorphic multinucleated cells that characterize these tumors are prelipoblastic mesenchymal cells. The possible significance of an interaction between these cells and mast cells is discussed.