Forty conjunctival biopsies from children suffering from metabolic diseases of the CNS were studied ultrastructurally. In 20 cases they were abnormal (8 mucopolysaccharidosis, 6 G M1 gangliosidosis, 4 infantile neuroaxonal dystrophy, 1 G M2 gangliosidosis, 1 late infantile ceroid lipofuscinosis). In the 20 remaining cases the biopsies did not show abnormalities. From these, 2 were proven as Leigh disease and Hallervorden-Spatz disease in which there is no conjunctival ultrastructural alterations; in 2 cases (one metachromatic leukodystrophy and one adrenoleukodystrophy) the results were false negative); in 4 clinicaly suspected cases of late infantile ceroid lipofuscinosis no alterations were observed and the results were not conclusive. In the remaining 12 cases the negatives allowed to rule out lysosomal disorders. The ultrastructural study of the conjunctival biopsy is an important tool for the diagnosis of neurological metabolic diseases of children mainly when sophisticated biochemical procedures are not available.