A cross-sectional epidemiological study was conducted to assess the prevalence and etiology of mental retardation (MR) among children under 14 years of age. The definition of MR and the criteria for diagnosis were adopted from WHO (1985). The IQ/DQ was evaluated with standardized psychological tests. The etiology of MR was specified by medical, genetic and psychosocial studies. A total of 862 retarded children were found among 85,170 surveyed. The overall adjusted prevalence rate was 1.20%. The percentage of mild, moderate, severe and profound MR was 60.6%, 22.7%, 9.6%, and 7.1%, respectively. Specific causes were found in 673 cases (79.1%). The etiology was unknown in 21.9% children. Timing of the causes showed prenatal in 34% perinatal 11.9%, postnatal 33%. Biomedical causes consisted of 89.6% and sociocultural factors, 10.4%. Among the prenatal factors, genetic diseases ranked first and manifested as chromosomal abnormalities and inborn errors of metabolism. Among the postnatal causes, sequelae of CNS infections and other forms of acquired brain insult were most important. Most children of socio-cultural MR were at school age and of the mild category. Our results show that most MR cases are preventable.