Anaphylactic IgE-mediated reactions to Hymenoptera stings vary in their severity for reasons that are not clear. We investigated patients with a history of systemic anaphylatic reactions to honeybee or wasp stings. Nine (75%) of 12 patients with raised tryptase concentrations but only 28 (28%) of 102 patients with lower tryptase concentrations, had a history of severe sting reactions (p=0.004). Raised baseline serum concentrations of mast-cell tryptase and mastocytosis are potential risk factors for severe allergic reactions to Hymenoptera venom.