Abstract Background Several studies have reported in vitro cross‐reactivity between wheat and barley. However, evidence regarding the clinical cross‐reactivity of wheat and barley is limited. This study examined the clinical cross‐reactivity of barley and wheat among children with immediate‐type wheat allergies. Methods We examined the threshold dose of a wheat oral food challenge for wheat‐allergic children. We examined the reactivity of barley, and the oral food challenges of barley tea and barley rice were implemented as needed. We measured the specific immunoglobulin E (sIgE) levels in wheat, ω‐5 gliadin, and barley. Results We evaluated 53 children (39 [74%] boys) with a median age of 6.6 years. Among them, 39 (74%) patients had a history of anaphylaxis to wheat. The median wheat‐, barley‐, and ω‐5 gliadin‐sIgE levels were 57.3, 12.1, and 3.2 kU A /L, respectively. Twelve patients reacted to barley tea (1.8 mg), 14 reacted to barley rice (220–440 mg), and 27 were tolerant to barley tea and barley rice. Barley‐allergic patients had significantly higher wheat‐ and ω‐5 gliadin‐ and barley‐sIgE levels and significantly lower threshold doses of wheat than barley‐tolerant patients. Omega‐5 gliadin‐sIgE was the most useful predictor of barley allergy among wheat‐allergic patients; the ω‐5 gliadin‐sIgE 95% positive predictive value for barley allergy was 4.6 kU A /L. Conclusions Half of wheat‐allergic children reacted to barley. A lower threshold dose of wheat is related to cross‐reactive barley allergies. Omega‐5 gliadin‐sIgE predicts cross‐reactive barley allergy in children allergic to wheat. Clinical cross‐reactivity to barley should be considered in the management of wheat‐allergic children.