Ultrastructural study of a case of benign lipoblastomatosis revealed a cellular spectrum which included undifferentiated mesenchymal cells, stellate myxoid cells, fibroblasts, lipoblasts, lipocytes and many intermediate forms. The uni- and multivacuolated fat cells contained membrane and non-membrane bound cytoplasmic vacuoles, predominatly of saturated lipid. Their cytoplasm also contained simple-structured mitochondria devoid of intramatrical or crystalloid bodies. The fine structure of the cellular components of lipoblastomatosis supports the concept that this is a neoplasm related to fetal white fat rather than to brown fat.