One of the main objectives of studying intellectual disability (ID) in children is to explore its causes. A specific aetiological diagnosis is important in determining the prognosis, nature and extent of services needed to support affected children. Aetiology and risk factors in mild ID were studied in a cohort of longitudinally followed children (6-10 years of age, n=40) in four population groups in and around Lahore, Pakistan. The overall prevalence of mild ID was 6.2%. In 22% of the cases the onset of mild ID was prenatal with small for gestational age and multifactorial inheritance as the main underlying factors. During the postnatal period (28% of the cases), social deprivation and malnutrition were the major causes of ID. In a substantial proportion of the cases (50%), the cause of ID could not be traced. The present study indicates a clear relationship of mild ID with prenatal and postnatal malnutrition and social deprivation. Two independent variables, maternal illiteracy and small head circumference at birth, showed a clear association with the development of mild mental disability among children in the study population.