ABSTRACT.One hundred and twenty‐nine children in the 8‐ to 12‐year‐old age group in Cardiff were ascertained to have mild mental deficiency. Detailed obstetric data were obtained which indicated that nearly 69% of the children were associated with adverse obstetric factors. Fifty‐five per cent had the combined pathogenetic risk factors of pre‐, peri‐ and neonatal adversities. Forty‐two per cent of the cases had non‐optimal perinatal factors and 45% were considered to have prenatal causation. There were four children with chromosomal anomalies. The various adverse obstetric influences are dis