Ultrastructural examination of two biopsy cases of tuberous sclerosis, a subependymal tumor and a cortical tuber, revealed giant cells with astrocytic features in both cases. These included multipolar processes containing glial filaments, glycogen, and membrane-bound dense bodies, as well as formation of hemidesmosomes with pia and vascular basement membranes. Megamitochondria were also seen within glial processes. In addition to the numerous glial-glial contacts encountered in both cases, rare neuroglial junctions suggesting aberrant synapse formation were observed within the cortical tuber. This observation, in concert with the organization of multiple small neurites along large, aberrant astrocyte processes found within the tuber, mimics features of neuroembryonic development described in lower mammals and nonhuman primates. Such a parallel in ultrastructural morphology supports a primarily dysplastic nature for the lesions in tuberous sclerosis.