Aztreonam for Inhalation Solution vs Tobramycin Inhalation Solution in Patients With Cystic Fibrosis & Pseudomonas Aeruginosa

Inclusion Criteria: * Males or females aged 6 years and older * Subjects with CF as diagnosed by one of the following: documented sweat chloride \>= 60 mEq/L by quantitative pilocarpine iontophoresis test, or documented sweat sodium \>= 60 mmol/L, or 2 well characterized genetic mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene, or abnormal nasal potential difference with accompanying symptoms characteristic of CF * Documented PA in an expectorated sputum or throat swab culture within 3 months prior to Visit 1 or at Visit 1 * Subjects must be able to provide written informed consent/assent prior to any study related procedures; parent/guardian must be able to give written informed consent as necessary prior to any study related procedure * Subjects must have received previous treatment with aerosolized antibiotics without demonstration of drug intolerance * FEV1 \<= 75% predicted at Visit 1 * Ability to perform reproducible pulmonary function tests * Chest radiograph at Visit 1 without significant acute findings (eg, infiltrates \[lobar or diffuse interstitial\], pleural effusion, pneumothorax); or chest radiograph or magnetic resonance image (MRI) obtained within the 180 days prior to Visit 1 without acute findings and no significant intercurrent illness; chronic, stable findings (eg, chronic scarring or atelectasis) are allowed Exclusion Criteria: * Current use of oral corticosteroids in doses exceeding the equivalent of 10 mg prednisone a day or 20 mg prednisone every other day * History of sputum or throat swab culture yielding B. cepacia in the previous 2 years * Current requirement for daily continuous oxygen supplementation or requirement for more than 2 L/minute at night * Administration of any investigational drug or device within 28 days of Visit 1 or within 6 half-lives of the investigational drug (whichever is longer) * Known local or systemic hypersensitivity to monobactam antibiotics * Known allergies/intolerance to tobramycin * Inability to tolerate inhalation of a short acting beta agonist * Changes in or initiation of chronic azithromycin treatment within 28 days prior to Visit 1 * Administration of antipseudomonal antibiotics by inhalation, intravenous or oral routes within the 14 days prior to Randomization/Visit 2 * Changes in antimicrobial, bronchodilator (BD), dornase alfa, or corticosteroid medications within 7 days prior to Visit 1 * Changes in physiotherapy technique or schedule within 7 days prior to Visit 1 * History of lung transplantation * Abnormal renal or hepatic function or serum chemistry at Visit 1, defined as aspartate aminotransferase (AST), alanine aminotransferase (ALT) \> 5 times upper limit of normal range (ULN) or creatinine \> 2 times ULN * Positive pregnancy test at Visit 1; all women of childbearing potential will be tested * Female of childbearing potential who is lactating or is not (in the opinion of the investigator) practicing an acceptable method of birth control; female subjects who utilize hormonal contraceptives as one of their birth control methods must have used the same method for at least 3 months before study dosing * Any serious or active medical or psychiatric illness, which in the opinion of the investigator, would interfere with patient treatment, assessment, or compliance with the protocol