Progressive mental retardation is the main clinical symptom in aspartylglucosaminuria. The intellectual functions of 116 patients were studied. Based on cross‐sectional test results, an average longitudinal developmental profile consisted of three parts: a period of abnormally slow but positive development in childhood, a gradual loss of reached skills after the age of 13–16 years and a rapid decline after the age of 25–28 years. Only nine young patients had a developmental age higher than six years, and two higher than seven years. The mean age of the female group was seven years higher than that of the males, but the mean developmental ages were the same for both g