We report a case of a preterm infant who developed cow's milk allergy. This male infant presented with recurrent ascites and was successfully treated with donated breast milk. He was born at 24 weeks' gestation with a birthweight of 506 g. From day 20, infant formula, soy protein-based formula, and casein-hydrolyzed formula were used due to insufficient maternal lactation. This resulted in abdominal distention, generalized edema, and recurrent ascites. We diagnosed him with cow's milk allergy since these symptoms improved on exclusive breast milk feeding. No recurrence of symptoms occurred when donated breast milk was used in combination with the mother's own milk. Ascites should be regarded as a clinical symptom of neonatal cow's milk allergy. Donated breast milk may be effective in the treatment of the allergy if breastfeeding is not available.