Over a period of 4 years, 88 infants with cow's milk protein intolerance (CMPI) were followed prospectively in order to evaluate the persistence of CMPI and its relationship between either serum IgE levels or RAST results for cow's milk. After exclusion of lactose intolerance, two positive cow's milk elimination challenge tests were considered diagnostic for CMPI. At the age of 1, 2, 3 and 4 years respectively, 85%, 78%, 49% and 33% of the children still were cow's milk intolerant. Initial serum values of IgE greater than or equal to 10 kU/l indicated a late development of tolerance to cow's milk proteins. At the age of 4 years, 90% of infants with initial IgE levels less than 10 kU/l had become tolerant to cow's milk while this was the case for only 47% of infants with initial IgE levels greater than or equal to 10 kU/l. Initial RAST results for cow's milk bore no obvious relationship to outcome.