Two cases of lipoblastomatosis were examined by electron microscopy. In each there was a spectrum of adipose cell differentiation from mature adipocytes to undifferentiated mesenchymal cells. The cells had various shapes and were loosely related to one another and to small blood vessels in a stroma containing sparse collagen and proteoglycan granules. Adipocytes and lipoblasts had a discrete basal lamina but this was absent in immature mesenchymal cells, which had prominent intermediate (10-nm) microfilaments and few or no lipid droplets. Neither case had foci resembling brown adipose tissue or hibernoma (large polygonal cells closely apposed to one another and to capillaries with densely packed pleomorphic mitochondria and focal undulating plasmalemmal invaginations). Both the ultrastructure of lipoblastomatosis and its characteristic loose lobular arrangement by light microscopy closely resemble human fetal "white" adipose tissue.