We hypothesized that the introduction of a practice guideline for penicillin skin testing would increase the appropriateness of skin testing and reduce antibiotic costs for patients with a history of penicillin allergy who have infections caused by penicillin-susceptible pathogens. We measured the appropriateness of skin testing and daily antibiotic costs before and after the introduction of a guideline for penicillin skin testing. For patients who had negative results of skin testing and were subsequently treated with a penicillin instead of an alternative antibiotic, we calculated the difference between the actual costs and the projected costs of continuing alternative antibiotics without skin testing. After the guideline was introduced, appropriateness of skin testing increased from 17% to 64%, but daily antibiotic costs did not change. For patients who had negative results of skin testing and who were subsequently treated with a penicillin, there was no difference between actual costs and the projected costs if they had not been skin tested. We conclude that introduction of a guideline for penicillin skin testing increases the percentage of eligible patients who have a skin test, and it does so without increasing costs.