In cancer patients, hypersensitivity reactions to adjunctive medications are easily mistaken for cytostatic toxicities. We report on three patients with systemic reactions (flush, dyspnea, tachycardia, hypotension, back pain) to a lipid emulsion containing long chain fatty acids (LCT). Reexposure to LCT and exposure to MCT (medium chain fatty acids) solutions of slightly different composition--no soybean lecithin used as an emulsifier--were well tolerated. These data suggest that traces of soybean proteins are the allergenic agents. Therefore, hypersensitivity to concomitant medications, including parenteral nutrition, has to be considered in oncologic patients demonstrating severe systemic reactions to intravenous therapy.