Cow's milk protein allergy (CMPA) is the most common cause of allergy occurring in the first year of life due to infant formula or breast-milk of mothers who are drinking cow's milk or eating cow's milk products. Most children with allergic colitis are symptomatic in the first months, usually by 4 weeks. There are rare cases whom were sensitized prenatally and demonstrated symptoms in the first week, even in the first 2 days of life. The most common clinical sign of CMPA is bloody stool in a well-appearing infant. Gross bloody stool or fecal occult blood are also the common signs of necrotizing enterocolitis (NEC), especially in preterm infants with systemic instability. The treatment options are totally different so the clinician has to be very careful evaluating the patient. We report 5 preterm cases of CMPA, two of whom were siblings. Two of them presented with massive bloody stools and 3 of them presented with abdominal distension and fecal occult blood all of which were initially considered as NEC. Literature review of 20 cases with similar history is summarized as well.