A benign lipoblastoma and a myxoid liposarcoma were studied by light and electron microscopy. Both of these neoplasms had prominent plexiform vascular networks, early acquisition of fat by vascular pericytes, and progressive accumulation of fat by cells located away from the vasculature. Their component cells had investing basal laminae, pinocytotic vesicles, and cytoplasmic glycogen stores as well as cytoplasmic lipid. The process of neoplastic lipogenesis and the structural features of the neoplastic cells in both neoplasms resembled those of developing non-neoplastic fat tissue. The benign lipoblastoma appears to be analogous to developing fat, while the myxoid liposarcoma appears to recapitulate the actively proliferating zone of developing fat. The relationship between proliferating cells and the plexiform vascular network in all three processes is emphasized. We hypothesize that the vascular pericyte serves as a source for new fat storing cells.