Results: 92.9% of the patients had an allergic reaction 6 years or longer before enrollment in the study. Of 272 challenges, 137 were skin-test positive with mild rash in 9 patients (6.6%), and 135 were skin-test negative with similar allergic reaction in 5 (3.7%) (P =.29). At follow-up, 3 of 55 patients (5.5%) who were given a full treatment course of penicillin developed mild skin eruption.