Clinical manifestations of cow's milk allergy rarely occur in the first days after birth. We report on a newborn presenting with hemorrhagic meconium in the first hour of life followed by bloody diarrhea in the next few days. At day 14, an elevated total IgE, specific IgE to cow's milk and an eosinophilia in peripheral blood were found. Symptoms disappeared when the milk feed was changed to an extensively hydrolyzed casein-formula. Two challenges with cow's milk formula (on day 30 and at 7 months of age) were followed by recurrence of vomiting, watery diarrhea and failure to thrive. At the age of 17 months cow's milk was tolerated well. Although other pathogenetic mechanisms cannot completely be ruled out, there is strong evidence that cow's milk allergy--induced by intra-uterine sensitization--explains the symptoms in our patient. In conclusion, cow's milk allergy can occur even in the first days of life, and our clinical observation supports the concept of intra-uterine sensitization to allergens.