The immunogenicity, allergenicity, and cross-reactivity of aztreonam were investigated in 19 patients with cystic fibrosis (CF) who are allergic to beta-lactam antibiotics. Skin tests with benzyl-penicilloyl polylysine (BPO), minor determinant mixture, and the drug responsible for the previous allergic reaction were positive for 26%, 53%, and 79% of the patients, respectively. Serum IgG, but not IgE, antibodies to BPO were detected in nine of 14 patients. Eighteen patients whose skin tests with aztreonam were negative were treated. One developed bronchospasm. The others tolerated aztreonam, with an improvement in clinical score greater than or equal to 1 month after treatment (P less than .001). One patient without previous exposure had positive aztreonam skin tests and was later treated uneventfully after iv desensitization. Treatment with aztreonam did not result in IgG or IgE antibody responses in vitro. However, two patients had anaphylactic reactions on reexposure. In patients with CF, allergy to beta-lactam antibiotics is primarily drug-specific. But despite reduced immunogenicity and cross-reactivity, aztreonam should be administered cautiously to patients with CF who are allergic to other beta-lactam antibiotics because it is potentially allergenic with repeated use.