The causal association between cow's milk allergy (CMA) and constipation is not well established. Some guidelines describe constipation as a possible symptom of CMA, while others do not mention it. We conducted a literature review and found 10 prospective clinical trials. In all of them, an oral food challenge was performed, and 2 of them were randomized. These studies reported that a cow's milk (CM) protein-free diet has a beneficial effect on constipation, with a rate of successful outcomes ranging from 28 to 78%. The hypothetic pathogenic mechanism lies in increased anal pressure at rest, probably caused by allergic inflammation of the internal sphincter area due to mucosal eosinophil and mast cell infiltration. Eighty percent of patients reach tolerance within 1 year after the diagnosis of CMA-related constipation. We believe that a CM-free diet for 2-4 weeks should be proposed for children with chronic functional constipation, even if it is not severe or resistant to laxatives.