We studied antibodies to cow's milk, beta-lactoglobulin and gliadin with enzyme-linked immunosorbent assay in the sera of 91 children with insulin dependent diabetes mellitus (IDDM); 36 of them were newly diagnosed. The children with newly diagnosed IDDM had significantly higher levels of IgA antibodies to cow's milk and to beta-lactoglobulin, and IgG antibodies to beta-lactoglobulin than 100 age-matched controls. We infer that either the pattern of cow's milk consumption is altered in children who will have IDDM, or their immunological reactivity to proteins in cow's milk is enhanced, or the permeability of their intestines to cow's milk protein is higher than normal.