Objective: To determine the antigenicity, nutritional adequacy, and growth-promoting efficacy of protein hydrolysate or amino acid–derived formulas in infants with cow milk allergy. Study design: Several protein hydrolysate or amino acid–derived formulas were graded for β-lactoglobulin content and skin reactivity in 74 atopic children with cow milk allergy proved by a double-blind, placebo-controlled challenge. A randomized, prospective follow-up study of 9 months included 22 infants with a mean age of 6 months (95% confidence interval, 4 to 7), who were fed an extensively hydrolyzed whey formula (group W e), and 23 infants with a mean age of 7 (95% confidence interval, 4 to 7) months, who were given an amino acid–derived formula (group AA). Results: Both formulas were clinically and biochemically tolerated. The mean concentration of essential amino acids in plasma was lower in group W e but higher in group AA compared with values for breast-fed control infants ( p = 0.001). There was a different trend between the groups in weight ( p = 0.09) and length ( p = 0.006). Growth was promoted in group AA during the follow-up; it was constant during the first months, followed by a gradual decline in rate in group W e. In both groups, atopic eczema improved significantly and progressively, and a downward trend was found in serum total and milk-specific IgE concentrations, proving the efficacy of both formulas. Conclusions: Extensively hydrolyzed formulas are safe and effective for most infants; an amino acid–derived formula may be preferable for infants with multiple food allergies, especially for the maintenance of normal growth. (J P EDIATR 1995; 127:550-7)