Background: Cow's milk allergy (CMA) affects 2.5% of children less than 2 years of age, but about 80% become clinically tolerant within the first 3 years of life. Casein is one of the major allergens responsible for CMA and seems to play an important role in persistent allergy. Previous studies on egg allergy suggested that linear epitopes are associated with long-lasting food allergy. Objective: The aim of the study was to identify IgE- and IgG-binding epitopes on ⍺_s1-casein and to determine whether the patterns of epitope recognition are associated with the natural history of CMA. Methods: According to the known amino acid (AA) sequence, 96 overlapping decapeptides representing the entire length of ⍺_s1-casein were synthesized on a cellulose-derived membrane. Sera from 24 children with milk allergy were used to identify IgE- and IgG-binding epitopes. Results: Six major and 3 minor IgE-binding, as well as 5 major and 1 minor IgG-binding, regions on ⍺_s1-casein were identified. Two IgE-binding regions (AA 69-78 and AA 173-194) were recognized by the majority of patients over 9 years of age with persistent allergy (67% and 100%, respectively) but by none of the children less than 3 years of age who are likely to outgrow CMA. No differences in IgG binding between the groups were observed. Conclusion: There appears to be a difference in epitope recognition between patients with different natural histories of CMA. Screening for IgE antibodies to these epitopes may be useful in identifying children who will have persistent milk hypersensitivity. (J Allergy Clin Immunol 2001;107:379-83.)