Twenty-one patients have been admitted to the Yale-New Haven Hospital during the past 16 years with the diagnosis of gastrointestinal milk allergy. All of the patients had gross or occult blood in their stools at some time, and most stools contained mucus. The predominant complaint was chronic diarrhea which varied from the passage of three to four semi-formed stools to 20 watery movements per day. Seven infants developed cardiovascular collapse after milk challenge and two of these had gastrointestinal hemorrhage as well. Sigmoidoscopy and rectal biopsy performed on eight children revealed changes of colitis which reverted to normal after the elimination of milk from the diet. Lactose tolerance tests were normal in seven of the eight. There was no correlation between the degree of symptomatology in infancy and the subsequent development of cutaneous or respiratory allergy. It is suggested that milk-induced colitis be considered a histologically documented and distinct entity.