A study was performed in infants under the age of 12 months born during 1974 and admitted to St. Göran's Children's Hospital with symptoms suggestive of cow's milk allergy (CMA). The aims of the study were to determine the role of early exposure to cow's milk formulas as a predisposing factor to CMA and to estimate the incidence of CMA in infancy. Twenty-five infants fulfilled the criteria for CMA. Available records were reviewed and a careful history was obtained from the mothers on two occasions. The patient group was compared with a control group. Sixteen of the 25 infants were exposed to cow's milk protein during their first week in the nursery for newborns, 6 were exposed before the end of the fourth week of life, and 3 infants were apparently not exposed. All infants were breast fed 3 to 26 weeks before re-exposure and occurrence of symptoms. Infants with CMA were given cow's milk formulas during their first 4 weeks of life significantly more often than infants in the control group (p less than 0.01). The incidence of CMA was approximately 1 : 200. The first 4 weeks after birth seem to be a particularly vulnerable period. Hence, in order to prevent CMA, infant formula should not be given--even occasionally--during this period.