Background: Aztreonam lysinate for inhalation (AI) is a novel monobactam formulation being investigated for pulmonary Pseudomonas aeruginosa infections in patients with cystic fibrosis (CF). Methods: Pre-clinical studies investigated the pre- and post-nebulization activity of AI and its activity in the presence of CF sputum. A double-blind, placebo-controlled, dose-escalation trial determined pharmacokinetics and tolerability of AI in subjects with CF. Single daily escalating doses of AI 75, 150, or 225 mg, or placebo were self-administered using an eFlow® Electronic Nebulizer. Sputum samples were collected up to 4 hr and blood samples up to 8 hr post-dose. Results: AI activity against multiple CF isolates was retained after nebulization via eFlow, and activity was not inhibited by CF sputum. All 12 adult subjects and 11/12 adolescents tolerated all AI doses. One patient had an asymptomatic FEV 1 decrease >20% with the 150 mg dose. Median aztreonam sputum concentrations in adults 10 min after AI 75, 150, and 225 mg were 383, 879, and 985 μg/g, respectively. Median sputum concentrations in adolescents 10 min after AI 75, 150, and 225 mg were 324, 387, and 260 μg/g, respectively. Systemic exposure to AI was low. Plasma pharmacokinetics in adults receiving AI 75 mg were Cmax = 419 ng/g, Tmax = 0.99 hr, t 1/2 = 2.1 hr. Aztreonam concentrations in sputum were at or above the MIC50 for at least 4 hr post-dose. Conclusion: These data support the continued development of AI for treatment of pulmonary infections in patients with CF. © 2006 Wiley-Liss, Inc.