Among the many types of adverse effects of drugs, allergic reactions constitute a very significant minority, with respect to both their frequency and sometimes serious consequences. To keep the term "drug allergy" meaningful, it should be limited to those adverse drug reactions that are based on immune mechanisms or that can reasonably be presumed to have this basis. Pseudoallergic drug reactions, which will also be considered in this issue, have similar clinical manifestations and some common pathogenetic mechanisms, but the initiating event does not appear to involve a reaction between the drug or a drug metabolite and specific antibodies. Clinically, drug allergy is commonly observed in nonatopic as well as atopic people. Innumerable drugs have been reported to produce these types of reactions, but in many instances drug metabolites may be the actual culprits. Clinical manifestations of drug allergy also are legion. Unfortunately, essentially none of these is unique or specific for drug allergy, but it is important for clinicians to think of this very treatable condition along with other diagnostic possibilities. It is convenient and helpful to classify allergic reactions to drugs according to Gell and Coombs' four main types of hypersensitivity processes, but in many instances more than one mechanism may be involved, just as immune responses to most antigens generally are complex. Type I reactions are generally immunoglobulin (Ig) E-mediated, and clinical manifestations include urticaria, angioedema, respiratory symptoms, and anaphylaxis. Pseudoallergic reactions of the latter type are called anaphylactoid.(ABSTRACT TRUNCATED AT 250 WORDS)