Autopsy reports are presented of three cases of a rapidly progressive encephalopathy with clinical onset around one year of age, early amaurosis, and microcephaly. Convulsions were few or absent. The disorder led to an extraordinary degree of brain atrophy, due to total loss of neurons from the cerebral and cerebellar cortex, and an advanced degree of neuronal destruction in most subcortical centres. The giant cells of Betz and the primary motor and sensory neurons were notable exceptions. The surviving neurons, other neuroectodermal cells, and a number of extraneural cell types showed accumulation of autofluorescent sudanophilic granules, resistant to lipid solvents, in their cytoplasm. Ultrastructurally, these granules were of the residual body type, consisting of membrane-bound conglomerations of spherical globules 0.2–0.5 μm in diameter, with a homogeneous, finely granular internal structure. These lesions were associated with a pronounced astrocytic and mesenchymal reaction with the presence of large numbers of phagocytic cells in the grey matter of the CNS and, to a lesser extent, in other tissues. In addition, there was almost total loss of myelin from the brain, apparently due to Wallerian degeneration. The characteristic clinical, histological and ultrastructural features differentiate this condition from other progressive encephalopathies of the age group in question, including the late infantile type of the Batten-Vogt syndrome. Recent ultrastructural and biochemical findings indicate that the disease of our patients is identical with the progressive encephalopathy with disturbed polyunsaturated fat metabolism described by Hagberget al. (1968).