BACKGROUND: Open-label, parallel-group, international trial comparing aztreonam for inhalation solution (AZLI) and tobramycin nebulizer solution (TNS) for cystic fibrosis patients with airway Pseudomonas aeruginosa. METHODS: 273 patients (≥6years); randomized to three 28-day courses (AZLI 75mg [three-times/day] or TNS 300mg [twice/day]); 28 off-days separated each course. RESULTS: 268 patients were treated (AZLI/TNS: 136/132). Mean baseline FEV(1) was 52% predicted. Mean relative changes after 1 course (AZLI: 8.35%; TNS: 0.55%; p<0.001) and mean actual changes across 3 courses (AZLI: 2.05%; TNS: -0.66%; p=0.002) indicated AZLI statistical superiority vs. TNS. AZLI-treated patients had fewer respiratory hospitalizations (p=0.044) and respiratory events requiring additional antipseudomonal antibiotics (p=0.004); both treatments were well tolerated. 133 patients received 1 to 3 courses of AZLI treatment in the open-label extension-period (28-day courses separated by 28days off-treatment); lung function improvements were comparable regardless of whether patients had received TNS or AZLI in the preceding comparative period. CONCLUSIONS: AZLI demonstrated statistical superiority in lung function and a reduction in acute pulmonary exacerbations compared to TNS over 3 treatment courses (ClinicalTrials.gov: NCT00757237).