Adverse effects of medications, most notably antimicrobials, are becoming increasingly common and raise difficult challenges in the area of clinical pattern definition (wide variety of symptoms, polypharmacy in many cases), diagnosis, and methodology (need for a rapid diagnosis, frequent obscurity of causative mechanisms, and less than ideal reliability of laboratory techniques). Sixty patients were treated by rush immunotherapy to one or more antimicrobials. The pretreatment evaluation included oriented history taking, skin tests, blood cell counts, IgE assays, and cell activation tests (basophils and lymphocytes). The results of this study confirm the usefulness of skin tests (intradermal, prick, or patch tests), which provided etiological orientation in 54 of the 60 cases. They also provide additional evidence of the lack of reliability of currently available in vitro tests (only 29 of the 60 tests were positive).