The etiology of severe mental retardation has not previously been studied in Jordan. In a group of 203 mainly severely mentally retarded children born 1975-1985, we found the etiology to be related to two main factors: (1) sequele from high perinatal morbidity or meningitis in infancy leading to a combination of severe mental retardation and cerebral palsy; (2) a high degree of intermarriage and a high frequency of retarded siblings indicating that genetic causes of MR are common. The results underscore the importance of further development of antenatal and perinatal care, especially for poor people and the need of genetic counselling.