The use of the sting challenge (SC) in insect allergy has brought into doubt the widely held belief that clinical data, specific IgE and the skin test are reliable tools in diagnosis and decision making with regard to therapy. We subjected ninety patients, known to have a systemic reaction to SC, in the Intensive Care Unit. Of these patients, 28% once more showed a systemic reaction; 72% had only a local reaction. No statistically significant difference could be demonstrated between positive and negative SC patients with regard to age; sex; the presence of allergic disease(s)—other than allergy to insects—and the severity of the preceding systemic reaction; the sting site and the related time interval; or specific IgE, IgG1, IgG4, their ratios and the skin test. A negative skin test and/or specific IgE, however, does not exclude the possibility of a recurrent systemic reaction. The long-term value of the SC appeared to be good since the results of the revision SC remained unchanged. Skin test and specific IgE are useful as diagnostic tools in establishing the insect concerned. SC provides information on the likelihood of a recurrent systemic reaction.