Purpose of study : the present research was carried out to study the prevalence of mental retardation among the youth and children of Savojbolagh region, and to study its probable causes. Method: the study comprised two phases. In the first phase, 1229 girls and boys, aged 5 to 18, were selected from among the children and youth of the urban and rural areas of Savojbolagh, through cluster sampling. In order to assess the prevalence of mental retardation, the raven test was administered and a psychiatric review was done, as a result of which, mental retardation was detected in 55 subjects. In the second phase, the probable cases of mental retardation were examined through a questionnaire and urine and blood tests administered to the 55 subjects. Findings: the spread of the disorder was found to be 4.4% which is much higher than what is reported in other researches in Iran and elsewhere. As of the causes, the phenilketonuria complication was 3.6% in the studied sample, which is much higher than what is reported globally, and which therefore calls for more extensive research in the field. In addition, intermarriage, and repeated labors within short intervals seem to have provided grounds for the disorder. The additional findings reveal that postnatal jaundice has been high among this group of children. This also requires further multidimentional researches.