Cross-reactivity between aztreonam and penicillins is poor, but clinical tolerance of aztreonam has been assessed, by means of tolerance challenge tests, only in a few groups of penicillin-allergic patients. The aim of this study is to evaluate the tolerability of aztreonam in a large group of beta-lactam-allergic patients. We studied all patients (greater than 14 years of age), with a clinical history of immediate reactions to any beta-lactam and with positive immediate-type skin tests and/or positive specific IgE to any of the studied beta-lactam; they were studied by means of: skin prick and intradermal tests with penicilloyl polylysine, minor determinant mixture, semisynthetic penicillins, cephalosporins, aztreonam and imipenem; detection of specific IgE to penicillin G, penicillin V, ampicillin, amoxicillin, cefaclor and ceftriaxone. Patients with negative immediate-type skin tests with aztreonam then underwent a graded intramuscular challenge. Forty-five patients (mean age 46.1 +/- 15.2 years), 27 females and 18 males, had positive skin tests and/or specific IgE to at least one of the studied beta-lactams. The most involved drugs were amoxicillin (23 cases), ampicillin (9 cases), penicillin G (8 cases) and other beta-lactams in the remaining cases. The most frequent reactions were anaphylaxis (27 cases) and urticaria (15 cases). All patients had negative intradermal tests with aztreonam and all patients tolerated the intramuscular graded challenge. Our data confirm the lack of cross-reactivity between beta-lactams and aztreonam. Immediate-type skin tests with aztreonam represent a simple and rapid diagnostic tool to establish tolerability in beta-lactam-allergic patients who urgently need this drug.