Up to 2.5% of infants are affected by cow milk hypersensitivity in the first two years of life, although most of these children will "outgrow" their reactivity within 2-3 years. Extensively hydrolyzed ("hypoallergenic") cow milk-based formulae are often recommended as a substitute formula and are generally well tolerated. However, a small minority of cow milk-allergic children experience allergic reactions to the hypoallergenic formulae as well. Utilizing inhibition-ELISAs and sera from patients with IgE-mediated cow milk allergy, we have identified residual protein fractions less than 20 kD in several of the extensively hydrolyzed cow milk-based formulae. Although many of the cow milk allergic children had positive skin prick tests [SPT] to one of the hydrolysate formulae (Nutramigen), the positive skin test result generally did not correlate with clinical reactivity, although the negative predictive value of the negative SPT was excellent. Children with IgE-mediated cow milk allergy and a positive skin prick test to the hypoallergenic formula should probably receive their first dose of the formula in a medical setting so that appropriate therapy can be administered in the unlikely event of an allergic reaction.