All cancer chemotherapeutic agents, except altretamine, the nitrosoureas, and dactinomycin, have produced at least an isolated instance of a HSR. Certain drugs, such as L-asparaginase and mitomycin (administered intravesically), cause HSRs of significant degree in approximately 10% of patients. All four types of HSRs are represented in the reactions produced by antitumor drugs, although Type I is the most common. Some of the Type I reactions are IgE-mediated, and others are probably mediated by nonspecific release of vasoactive substances from targets such as mast cells. It is possible to continue therapy with some drugs, despite a prior HSR, if the prophylactic measures outlined in Table 2 are taken. An example of this is provided by taxol in which the lengthening of the infusion time and the administration of preventive medication allowed some patients to continue taxol therapy. The mechanisms of the HSRs have been carefully assessed in only a minority of patients who sustained such toxicity. Such evaluation would increase our understanding of this form of drug toxicity and perhaps lead to means of effectively reducing the risk and severity.