Cow's milk (CM) allergy is the most common food allergy in children, affecting 2-3% of infants. In formula-fed infants with CM allergy, extensively hydrolyzed or amino acid-based infant formulas are typically recommended. These formulas are expensive and not palatable. For older patients with CM allergy, the standard of care is avoidance of all CM products. The investigators cared for a CM allergic patient who was able to tolerate partially hydrolyzed whey formula (pHWF), which tastes better and is less expensive than extensively hydrolyzed or amino-acid based formulas. There are likely other subjects who could similarly tolerate pHWF. Furthermore, it is possible that taking pHWF could accelerate tolerance of CM. The investigators aim to identify characteristics of CM allergic subjects who can tolerate pHWF and assess the degree to which taking pHWF accelerates CM tolerance.

Specifically, subjects who have CM allergy will be enrolled. The investigators will perform an oral food challenge (OFC) to pHWF in each subject. Subjects who pass the challenge will be placed on pHWF for 2 years. Subjects who do not pass the challenge will receive routine care. The investigators will perform allergy skin testing, specific IgE measurement, other serologic assays, and longitudinal OFCs to profile subject characteristics in the two groups at baseline and over time.

Inclusion Criteria:

• age 0.5-18 years old
• positive skin prick tests (SPT) or detectable serum milk specific IgE and a history of an allergic reaction to milk within 6 months before study screening; or milk-specific IgE level or SPT highly predictive for clinical reactivity (if ≤ 2 years old, sIgE >2 kU/L; if > 2 years old, a level >5 kU/L; SPT wheal diameter ≥ 3 mm)
• a clinical reaction during the cows milk oral food challenge

Exclusion Criteria:

• negative SPT and undetectable milk-specific sIgE
• unstable asthma, allergic rhinitis, or atopic dermatitis
• milk-induced eosinophilic gastroenteropathy
• recent reaction to partially hydrolyzed whey formula
• history of severe anaphylaxis with hypotension to cows milk
• participation in any interventional study for the treatment of food allergy in the 6 months prior to screening visit
• inhalant allergent immunotherapy that has not yet reached maintenance dosing
• inability to discontinue antihistamines for skin testing, OFC
• any systemic therapy which in the judgment of the investigator could be immunomodulatory in the 12 months prior to visit 1 (xolair, rituximab, chronic steroids, etc)
• investigational drug use 90 days prior to visit 1 or intention to use during study period
• the presence of any medical condition that the investigator deems incompatible with participation in the trial.
• unable to understand and speak English