Randomized Phase Study Design (US and EU): This was an open-label, multicenter, randomized, parallel group study. The study design consisted of 2 treatment arms of 28-day, intermittent, repeating treatment regimens: aztreonam for inhalation solution (AZLI) or tobramycin inhalation solution (TIS). The total study period was 26 weeks. The study schedule included 9 visits - Screening, Baseline, Day 14, Day 28, followed by visits every 28 days through the end of the study.