To investigate the incidence of cow's milk allergy as evidenced by milk challenge and the findings of endoscopic and immunohistochemical examinations in children with chronic and refractory constipation.Thirty-five study subjects (mean age, 8.3 +/- 3.3 years; range, 3-15 years; 17 girls) and 15 control subjects (mean age, 11.7 +/- 3.2 years; range, 2-15 years; 9 girls) were studied by colonoscopy and a 4-week cow's milk elimination and challenge.Lymphoid nodular hyperplasia was the most prominent endoscopic finding in half of the subjects (46%), mostly occurring patchily in the transverse colon. Histologic findings other than lymphoid accumulation and mildly increased density of eosinophils were few. During the milk elimination and with supportive medication, 83% of subjects remitted. Constipation and/or other gastrointestinal or skin symptoms relapsed only in one third (34%) during the cow's milk challenge, these having significantly higher densities of intraepithelial gammadelta + T cells ( P <.001) in the biopsy samples of the terminal ileum as compared with the control subjects.We were able to find formal evidence for the presence of cow's milk allergy in children with chronic constipation.