Trimethoprim/Sulfamethoxazole is the most effective medication used in both the treatment and prevention of Pneumocystis carinii pneumonia (PCP) in patients with HIV/AIDS. Its use, however, is accompanied by a high incidence of adverse reactions, especially fever, myalgia and rash (sulfa hypersensitivity). In a group of our patients, we have examined the clinical parameters at the time of onset of sulfa hypersensitivity, and the success of a desensitization protocol for this adverse event. We also have performed a comprehensive review of the literature on sulfa hypersensitivity and have compared our results to those previously reported in the literature. Our findings indicate that the sulfa hypersensitivity reaction is more likely to develop in patients with advanced disease and that desensitization can restore tolerability to the drug in approximately two thirds of those who attempt it.