Aspartylglycosaminuria (AGU, McKusick 20840) is a metabolic disorder affecting the catabolism of glycoproteins. It was first described in 1967, by Jenner and Pollitt, in two mentally retarded English siblings. Subsequently several cases were reported from Finland (Palo and Mattsson, 1970; Autio, 1972; Autio et al., 1973). Today the number of known cases is about 140, most of them Finnish or of Finnish origin (Aula et al., 1980). The incidence of AGU in Finland has been estimated to be approximately 1:26000 and the disease is inherited as an autosomal recessive trait (Autio et al., 1973). Clinical manifestations include progressive mental retardation, coarse gargoyle-like facial features, skeletal abnormalities and recurrent infections. Early development of the patients is usually normal, but by the age of 5-15 years they are already severely retarded (Autio, 1972; Autio et al., 1973). Morphologically AGU is a generalized storage disease (Haltia et al., 1975). Affected tissues show enlarged lysosomes. Vacuolization is a prominent feature of liver and nerve cells (Haltia et al., 1975) and of peripheral lymphocytes (Aula et al., 1975).