The object of this study was to ascertain the frequency of postinfectious cow's milk protein hypersensitivity (CMPH). Twenty-four infants less than 3 months old were included in the study. Following hospitalization for acute gastroenteritis, the infants were given a protein hydrolysate formula for a period of 6 weeks, after which an intestinal biopsy was performed. Thereafter, a milk challenge was given. The existence of CMPH was defined as a postchallenge reduction of one or more of the mucosal disaccharidases below the normal levels for our laboratory. A bacterial etiology of the gastroenteritis was found in 10. Nineteen infants had no adverse reaction to cow's milk after 6 weeks on a hypoallergenic formula. Only two could be confidently diagnosed as having developed secondary CMPH; both had been infected by Escherichia coli 0 111. One infant had primary CMPH and one extra-intestinal CMPH. The incidence of secondary CMPH with gastrointestinal manifestations in this series was considerably less than described elsewhere.