We describe a patient who presented shortly after birth with hyperkinetic behaviour, myoclonia, respiratory insufficiency and hepatosplenomegaly. Gaucher-like storage cells were found in bone marrow. A liver biopsy showed massive lysosomal storage morphologically different to that in known lipid storage disorders. Biochemically, the patient had partial deficiencies of beta-galactocerebrosidase, beta-glucocerebrosidase and ceramidase in skin fibroblast extracts, but the sphingomyelinase activity was normal. Glucosyl ceramide and ceramide were elevated in liver tissue. Loading of cultured fibroblasts with radioactive sphingolipid precursors indicated a profound defect in ceramide catabolism. Immunological studies in fibroblasts showed a total absence of cross-reacting material to sphingolipid activator protein 2 (SAP-2). The patient died at 16 weeks of age. The fetus from his mother's next pregnancy was similarly affected. The possibility that the disorder results from a primary defect at the level of SAP-2 is discussed. We have named this unique disorder SAP deficiency.