In an open clinical trial, aztreonam was administered repeatedly to 15 cystic fibrosis patients who were chronically infected with Pseudomonas aeruginosa and had previously had severe hypersensitivity reactions to other beta-lactam antibiotics, including anaphylactic shock, generalized urticaria, and drug-associated fever. After negative results in a skin-prick test and a lack of reaction to an intravenous test dose of aztreonam, the patients were treated with aztreonam (150 mg/[kg.d]) in combination with tobramycin (10-20 mg/[kg.d]) for 14-day periods at 3- to 4-month intervals. To date, a total of 56 course of aztreonam have been administered to these patients (three to six courses per patient), and no type 1 hypersensitivity reactions have occurred. However, as a result of drug-associated fever, the administration of aztreonam had to be discontinued in two of 15 cases. The remaining 13 patients have tolerated treatment well.