Aim: To study the aetiology of intellectual disability in patients presenting to hospital and the diagnostic yield of a standardized examination. Method: Over a 1-year period, the first three children presenting to the paediatric outpatients department (OPD) on 2 selected weekdays with developmental delay, suspected intellectual disability, or school failure were enrolled for study if they satisfied standard definitions of global developmental delay (GDD), or intellectual disability as tested by scales for Indian children: Developmental Assessment for Indian Infants, Binet Karnat Test, and the Vineland Social Maturity Scale (Malin's Adaptation). Detailed history, and physical and neurological examinations were recorded. An algorithmic approach to investigations was followed. Also, neuroimaging, thyroid function, electroencephalograph, karyotyping, and studies for fragile-X syndrome were conducted. Aetiological diagnosis was considered established only if clinical features were supported by investigations. Clinical features associated with a successful aetiological diagnosis were computed. Results: A total of 122 children were enrolled in a cross-sectional analytic study (mean age 43.5 mo [SD 40.66]; 84 males, 38 females). Of these, a definite aetiology could be assigned in 66 children (54.1%); 17 prenatal, 38 perinatal/neonatal, and 11 postneonatal. Factors associated with reaching a definite diagnosis included younger age at presentation, presence of seizures, microcephaly, adverse neonatal events, and abnormal motor signs. Clinical history and examination gave important clues to the aetiology in 89 (72.9%) patients. Neuroimaging was abnormal in 91 out of 114 children, with aetiological findings in 48 children. Interpretation: Perinatal/neonatal causes predominate as the cause of GDD or intellectual disability in India. The study highlights that a large majority of cases seen here were preventable.