Aspartylglucosaminuria (AGU) is a lysosomal storage disorder. The intrauterine and early development of individuals with AGU is usually normal but progressive intellectual disability develops from early childhood, speech and motor skills tend to be affected early, and affected individuals have dysmorphic features and abnormal skin. The progression of intellectual disability is slow at first but increases with age. Adults have severe /profound intellectual disability, seizures are present in 30% of affected individuals and psychiatric disorders in 20%. Affected adults may have seizures, movement disorders, osteoporosis, hypermobility, and loose skin. Macrocephaly may be present in children, while adults have microcephaly. The average life span of patients is usually less than 50 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.