At The lnstitute of Handicapped Children, 1100 mentally handicapped children with the Griffiths Mental Development Scale (GQ) less than 8O were investigated during a 5 year period between January 1981 and December 1985 to determine the cause of their intellectual deficit, to diagnose their condition and to assess their ability. Of 1100 children, 776 (69.6%) were diagnosed clinically. Cerebral palsy was the most common cause (22.5%) followed by microcephaly (14.6%), seizure disorder (12.4%) and chromosomal anomaly (4.4%). Preventable disorders such as cerebral palsy, chromosomal anomaly, metabolic disorder and endocrinologic disorder were found in 314 cases (28.5%). It is emphasized that prenatal care, fetal monitoring, chromosomal study and a screening test for hypothyroidism and phenylketonuria are indeed required for prevention. When the Griffiths Mental Development Scale was used to assess the children's ability, 96.5% required medical and paramedical treatment such as education and training. In order to assess a mentally handicapped child completely, specialists from various fields are needed to work as a team in an assessment unit where knowledge from all fields can be concentrated. In addition, sociefy as a whole should become more interested in these affairs.