Six infants are reported who developed an inflammatory proctocolitis in the first month of life while being breast fed exclusively. All has been born at term and had normal perinatal courses. None had growth failure or constitutional symptoms other than bloody diarrhea. No toxic, bacterial, viral, or parasitic cause was established. Rectal inflammation was suggested by the presence of fecal leukocytes and was confirmed by sigmoidoscopic observation of focal ulcerations, edema, and increased friability. Rectal biopsies demonstrated a wide spectrum of acute and chronic inflammatory changes. All infants responded clinically to initiation of feeding with either a hydrolyzed casein or a soy protein-based formula. Breast-feeding was subsequently resumed in five of the six infants; all experienced immediate recurrence of symptoms. Elimination of cow milk protein from the maternal diet led to tolerance of breast-feeding in two infants but there was no change in the other three. We believe that dietary protein-induced enterocolitis, previously reported in formula-fed infants, occurs occasionally in the exclusively breast-fed infant as well.