Rates of allergic skin reactions to commonly used drugs were estimated from data obtained on 22,227 consecutively monitored medical inpatients. A total of 57 drugs were implicated with skin reactions. Five or more reactions were attributed to each of 22 drugs (or drug groups). Many commonly used drugs did not appear to cause any allergic reactions. The study provides the practicing physician with drug-specific quantitative data that can be used to evaluate the causes of drug-induced rash, itching, or hives.