We evaluated 587 cases with generalized reactions to stings of Hymenoptera. Eighty of these patients and twenty-eight normal controls had radioallergosorbent tests (RAST) to venoms of honey bee, yellow jacket, hornet, wasp and to phospholipase A. Those patients with systemic reactions had a significantly greater frequency of positive RAST than normal controls (51.3%vs. 7.1%, P < 0.001). The frequency of atopy (asthma/rhinitis) in these 587 cases was only 22% and resembled the expected frequency in a general population. Asthmatic patients did not have an increased risk of developing systemic reactions to Hymenoptera stings. However, those asthmatic patients with systemic reactions to Hymenoptera stings had a significantly more severe anaphylactic reaction to a sting than non-asthmatics. These severe reactions were primarily manifested by acute dyspnoea, which appeared to represent a bronchospastic response to endogenous histamine release.