The European hornet, Vespa crabro, is a very large insect native to much of Europe and Asia and introduced into eastern North America. Four cases of allergic reactions to V. crabro stings are presented, one of which was fatal. V. crabro venom contains 2% protein and peptide. There are three major proteins: Phospholipase AB, antigen 5 and hyaluronidase. These proteins are structurally and antigenically related to those from other vespid wasps, especially Vespula yellow jackets. IgE antibodies from patients allergic to Vespula usually cross-react with V. crabro venom. In three of the four patients with known reactions to V. crabro venom antigen 5 appeared to be the most important allergen. Serum from one patient was most reactive with hyaluronidase. Phospholipase had relatively little IgE-binding activity, although it was the major protein in the venom. V. crabro venom is at least as closely related to Vespula venoms as Dolichovespula venoms are.