To investigate the risk of life-threatening reactions to future stings, we sequentially challenged 113 children (aged 2 to 17 years) allergic to insect stings with a sting by the relevant insect. The time interval between the challenges varied from 2 to 6 weeks. The history of the index stings was a large local reaction (LR) in 16% and a systemic reaction (SR) in 84% of the test subjects. On the first challenge, 76% had a normal LR, 11% a large LR, and 13% an SR. On the second challenge, 78% of the children had a normal LR, 5% a large LR, and 17% an SR. Thirty-nine of the untreated children were exposed to a field sting during the subsequent 3-year follow-up period. In comparison with other diagnostic evaluations such as skin-prick tests, determinations of specific IgE and IgG antibodies, and single-sting exposure, the dual sting challenge scheme appears to be the best predictor of reactions to subsequent stings. It also appears to be helpful in selecting patients with an uncertain sensitization status for venom immunotherapy.