The presence of minor neurological dysfunction is associated with behavioural and cognitive development at school age. We have previously shown a relation between minor neurological dysfunction and perinatal disorders, especially abnormal neonatal neurological condition. We have now investigated the relation between breastfeeding and long-term neurological development. We studied 135 breastfed (for > or = 3 weeks) and 391 formula-fed children, born at term in the University Hospital Groningen between 1975 and 1979. A standard neonatal neurological examination was used to classify the infants as normal (247), slightly abnormal (213), or frankly abnormal (66). At 9 years of age the children were reexamined. In 1993 their mothers were asked to complete a questionnaire about how the children were fed as infants. After adjustment for obstetric, perinatal, neonatal neurological, and social differences, a small advantageous effect of breastfeeding on neurological status at 9 years of age was found (odds ratio for neurological non-normality 0.54 [95% CI 0.30-0.97]). Although a retrospective design cannot lead to definite conclusions, our data suggest a beneficial effect of breast-feeding on postnatal neurological development. Longer-chain polyunsaturated fatty acids, which are present in breast-milk but not in most formula-milks, may have a role since they are vital for brain development.