This walnut OIT study is a randomized, blinded, placebo controlled study based on previous experience at The Children's Hospital of Philadelphia (CHOP) using OIT in food allergic subjects. CHOP will enroll 6 subjects (4 in the active treatment and 2 in the placebo treatment arms). Non-CHOP sites will enroll 24 subjects, making a total of 30 (20 in the active treatment and 10 in the placebo treatment arms) children and adults with walnut allergy and a second tree nut allergy. Subjects will be randomized in a 2:1 ratio into either an active treatment group (final dose 1500 mg walnut protein, n=20) or a placebo group (n=10). Subjects will undergo a one-day desensitization protocol designed to enable the subject to tolerate 6 mg of walnut protein or placebo (initial day escalation phase). After the initial escalation day achieving at least 1.5 mg and up to 6 mg of walnut protein or placebo, dosing build-up will occur every two weeks through dose 24 at 34 weeks. A maintenance dose will be given for 4 weeks followed by a 5 gram protein OFC to walnut and a 5 gram protein OFC to a second tree nut (at ~38 weeks), after which the study will be unblinded. Placebo subjects that fail the OFC will be crossed over to active treatment and escalated as described to the 1500 mg target dose. All subjects will be followed for a total of 142 weeks on active treatment which will be followed by an OFC (both on and then off therapy) to walnut and the second tree nut at the end of long-term maintenance therapy. Subjects that have a reduction in serum specific IgE to <5kU/L to walnut and the test tree nut before 142 weeks will be eligible for a tolerance OFC.