Cow's milk protein (CMP) allergy was investigated in 25 children (age-range 3 months to 11 years) with chronic constipation. A diagnosis of constipation was made on the basis of a history of painful elimination of hard stools for at least 1 month, whether or not associated with a reduced frequency of stools or soiling. The children were evaluated using clinical parameters and the following laboratory tests: total serum immunoglobulin E (IgE); specific IgE (radioallergosorbent test [RAST]) for whole cow's milk, alpha-lactoalbumin, beta-lactoglobulin, and a food group; and skin-prick tests with whole milk, alpha-lactoalbumin, beta-lactoglobulin, and casein. Following the evaluation, the children were submitted to a CMP-free diet for a period of 4 weeks. In seven patients (28%), constipation disappeared during the CMP-free diet and reappeared within 48-72 h following challenge with cow's milk. In two infants a rectal biopsy revealed allergic colitis and they therefore did not undergo the challenge. High serum levels of total IgE were observed in five of the children who showed a clinical improvement (71%), a positive skin-test in two (29%), and detectable specific IgE in two (29%). These results suggest that CMP allergy or intolerance should be considered as a cause of chronic refractory constipation in children, although the underlying mechanism still require further investigation.