In order to investigate the possible contribution of maternal emotional stress to the aetiology of infantile pyloric stenosis, 100 mothers of affected infants, 100 "normal" controls who had not sought medical advice at any time concerning feeding problems, and 50 mothers of children with spina bifida were interviewed. All completed the Life Events Inventory, which gives a quantitative measurement of stressful life events during a given period. In addition, the subject mothers and "normal" controls completed form A of the Eysenck Personality Inventory, the Multiple Affect Adjective Check list, and Linear Analogue Scales for the subjective measurement of satisfaction with feeding and distress caused by feeding problems. A highly significant difference was found between the number of items given a heavy weighting on the Life Events Inventory experienced by the subjects and both control groups. This was particularly marked during the last trimester of pregnancy. Personality assessment was very similar in all groups, but maternal anxiety scores were higher in the subjects than in the controls. It is concluded that hypertrophic pyloric stenosis occurs unduly frequently in infants of normal mothers who have been subjected to particularly stressful events during pregnancy.