Discordant results have been reported about the role of lymphocyte proliferation assays in patients with cow's milk allergy. We studied the peripheral blood mononuclear cell response of 10 children with cow's milk allergy by means of a lymphocyte proliferation test to determine the diagnostic value of this assay, the clinical tolerance of a new therapeutic hydrolyzed formula, and the evolution of lymphocyte proliferation after 3 months of a cow's milk-free diet with the hydrolyzed formula. The lymphocyte proliferation at the time of diagnosis in the patients with cow's milk allergy was not statistically different from the results in the control group. The proliferation test performed after 3 months of diet with the hydrolyzed formula and restriction of cow's milk protein showed that the cellular proliferation remained globally the same compared with the proliferation at the time of diagnosis. The hydrolyzed formula proteins induced a lower cellular proliferation than milk proteins in patients with cow's milk allergy. Our results suggest that the lymphocyte proliferation test cannot be recommended for diagnostic purposes. However, in patients with cow's milk allergy the proliferation test affirmed the absence of immunogenicity of the hydrolyzed formula because it induced no significant T-cell activation.