A prematurely born infant developed rectal blood loss several hours after birth, after his first formula feeding. Discontinuing the feeding resolved symptoms, but after resuming feeding rectal blood loss reappeared. There were no signs of necrotizing enterocolitis. Suspecting cow's milk allergy, the feeding was changed to a casein-based protein hydrolysate, without effect. Meanwhile, laboratory tests indicated cow's milk allergy. Symptoms only resolved after introducing an amino acid-based formula supporting a definite diagnosis of cow's milk-induced allergic colitis. This is the first description of a premature infant with symptoms of allergic colitis, appearing within hours after birth, suggestive of intrauterine sensitization. The exact mechanisms of sensitization remain obscure. CONCLUSION: Cow's milk-induced allergic colitis can occur after the first feed, even in a prematurely born neonate. This is most probably due to intrauterine sensitization, and should be included in the differential diagnosis of rectal blood loss.