Aspartylglucosaminuria (AGU) is a lysosomal storage disorder with a relatively uniform phenotype that includes progressive mental retardation from early childhood, coarse facial features and skeletal and connective tissue abnormalities. The intrauterine and early development of AGU patients is usually normal. During the first years of life, an increased tendency for recurrent upper respiratory infections is frequently observed with signs of disturbed development, most frequently a delay in speech development and motor clumsiness that usually becomes apparent between 2 and 5 years of age. The average life span of patients is 40 years. AGU is extremely rare except in the Finnish population where the carrier frequency is as high as 1 in 40 in some areas of the country.