Background: Some patients with a wheat allergy have been reported to show clinical cross-reactivity to barley. However, it is not clear whether the development of barley allergy in patients with a wheat allergy is due to cross-antigenicity between wheat and barley. In our study, we aimed to determine the clinical cross-reactivity and immunological cross-antigenicity of wheat and barley. Methods: We compared the results of barley oral food challenges (OFCs) before oral immunotherapy (OIT) for wheat with those after OIT in nine patients with a wheat allergy to estimate the clinical cross-reactivity of wheat and barley. Moreover, we performed enzyme-linked immunosorbent assay (ELISA) inhibition and immunoblotting inhibition using serum from seven patients allergic to wheat and barley. Results: Nine patients who had positive barley-OFC results performed before OIT for wheat were all negative on barley-OFC performed after OIT. In ELISA inhibition, preincubation of serum from patients allergic to wheat and barley with a high barley extract concentration inhibited binding of IgE to wheat extract by less than 10%. On the other hand, wheat and barley extracts equally inhibited binding to barley sIgE at high concentrations. In the immunoblotting inhibition test, the spots of wheat were inhibited but weakly by barley extracts, and most of the spots of barley were inhibited even by low concentrations of the wheat and barley extract. Conclusion: We showed that barley allergy associated with wheat allergy is caused by cross-reactivity from wheat. The OIT for wheat was one of the promising options for barley allergy.