BACKGROUND Elective penicillin skin testing in advance of acute antibiotic need and amoxicillin challenge in patients with negative skin test responses have not been evaluated. METHODS I reviewed 236 patients previously entered in a study of new penicillin reagents who received at least 1 prescription drug over a 2-year period. Antibiotic use, outpatient visit rate, and adverse reactions to antibiotics during the year before and after skin testing were evaluated. RESULTS Forty (17%) of the 236 subjects had positive responses. Antibiotic courses dispensed to the 236 subjects fell 28% from 779 the year before testing to 558 the year after testing. The total cost for antibiotics dispensed fell 32% from $17,211.88 to $11,648.27, with a 5.5% reduction in the average cost per antibiotic. Outpatient visit rate did not change but shifted from primary to specialty departments in subjects with both positive and negative skin test responses. In 93 subjects with negative skin test responses, a total of 188 therapeutic courses of penicillin during the year after testing resulted in 3 (3.2%) unrechallenged mild adverse reactions. Optional amoxicillin challenge in 146 of the subjects with negative skin test responses resulted in complaints of an adverse reaction in 6 of these subjects. Four of these received a penicillin analogue in the next year without reaction. CONCLUSIONS Elective penicillin skin testing done by an allergist was associated with unexpected declines in the number and cost of antibiotics used the year after testing but only modestly lowered the average cost per antibiotic. Adverse reactions to penicillins in subjects with negative skin test responses were infrequent, and amoxicillin challenge did not affect outcomes.