Allergic colitis (AC) typically develops in the first weeks or months of life and is characterized by the presence of red blood in the stools of healthy breastfed or formula fed infants. In this paper, we describe a case of rectal bleeding in monozygotic preterm twins that was resolved with the introduction of a cow's milk protein-free diet (CMPFD). The occurrence of this disorder in monozygotic twins raises the question as to whether the underlying abnormality in the immune regulation, which leads to poor acquisition of tolerance to cow's milk proteins, might be inherited or environmentally acquired. The case also highlights the use of the probiotic Lactobacillus GG (LGG) in the treatment of allergic colitis.