Five children with severe developmental delay had intractable fits of various types but tonic, often extensor, seizures were prominent from an early stage. Onset was in the neonatal period in 4 cases. EEGs were severely abnormal and showed a "burst-suppression" pattern in the first months of life. There were no metabolic or consistent neuroradiological abnormalities. A distinctive form of dentato-olivary dysplasia was found in all cases. Inferior olives were hook-shaped, coarse and lacking undulations, while dentate nuclei showed a compact arrangement of interconnected islands. The clinico-pathological findings form a novel nosological entity.