We present nine exclusively breast-fed, full-term infants with mild rectal bleeding due to proctocolitis. The mean age at the onset of symptoms was 5 weeks (range 1-8 weeks). Rectosigmoidoscopic examination was performed in all the children within 2 days after admission, showing inflammatory changes such as oedematous mucosa with petechial haemorrhages. Rectal mucosal biopsy specimens were obtained in eight cases and revealed intra-epithelial eosinophilic granulocytes in seven and a diffuse increase of eosinophils in the lamina propria in six. Allergy to cow's milk protein transferred to the infants via the breast milk was believed to be the cause of the inflammation. The intake of cow's milk protein was then restricted in seven mothers. Following this regimen, symptoms were relieved within 4 weeks in the six infants who were seen at follow up. One child recovered spontaneously without dietary restrictions. Considering the beneficial effect of the diet regimen in addition to the histological findings, allergy to cow's milk protein is possibly the aetiology of the proctocolitis seen in these nine exclusively breast-fed babies, although no challenge tests were performed to confirm this suspicion. CONCLUSIONS This report shows that proctocolitis occurs in exclusively breast-fed infants. It is speculated that allergy to cow's milk protein may have played a role in the pathogenesis.