The purpose of this study was to determine whether a new casein hydrolysate infant formula, Alimentum, could be administered safely to children with cow milk hypersensitivity. The formula was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and with a sensitive enzyme-linked inhibition immunoassay, and it was judged to be safe for clinical trials in children allergic to cow milk. Twenty-five such children underwent double-blind placebo-controlled oral food challenges with 10 gm of powdered cow milk and casein hydrolysate formula. All children were highly atopic and had positive skin prick reactions to cow milk. No patient reacted to placebo during a double-blind, placebo-controlled food challenge. Two patients lost their allergy to cow milk and did not react during the challenge; the remaining patients reacted with a variety of cutaneous, respiratory, and gastrointestinal symptoms within 15 to 90 minutes of challenge. All children tolerated the blinded challenge to the casein hydrolysate and were fed the hydrolysate openly without difficulty. We conclude that this casein hydrolysate is generally safe to feed to children with immediate hypersensitivity to cow milk. We recommend that all infant formulas promoted as "hypoallergenic" be tested in milk-allergic patients to assess their allergenic potential, in addition to standard nutritional evaluation and animal testing for antigenicity.