The results of a qualitative and quantitative neuromorphological and a biochemical analysis of a brain biopsy from a 7-year-old boy with an idiopathic type of Lennox-Gastaut syndrome (LGS) are reported. A disturbance in cerebral energy metabolism as expressed by the pyruvate pathway could be ruled out. Degenerative or ischemic lesions were not found. However, while the outer layers of the cortex were morphologically normal, the neurons of the inner layers (V and VI) showed a poor dendritic arborization and a diminished number of spines, which can be considered the basis of excessive excitability which in turn may further impair brain development. Thus, the neuromorphological findings suggest a causal rather then a consequential relation to LGS.