This clinical trial will attempt to determine whether we can improve clinical outcomes for patients with cystic fibrosis who are infected with a fungus called Aspergillus fumigatus.

The aim of this study is to determine whether antibiotic treatment directed against Aspergillus Fumigatus will be effective at preventing respiratory exacerbations and improving pulmonary function in patients with cystic fibrosis(CF) who are chronically colonized/infected with aspergillus. This aim will be accompanied by means of a randomized, double-blind, placebo-controlled clinical trial incorporating two parallel treatment arms.

Inclusion Criteria

• Diagnosis of CF as defined by two or more clinical features of CF and a documented sweat chloride greater than 60 mEq/L by quantitative pilocarpine iontophoresis test or a genotype showing two well characterized disease causing mutations
• Patient must be known to be chronically colonized with Aspergillus fumigatus.
• Patients must be clinically stable at randomization, no use of new inhaled, oral or intravenous antibiotics or oral or intravenous corticosteroids during the 14-day period prior to randomization.
• 6 years of age and older
• Patients must weigh at least 20 kg
• Post-menarche females must be using an effective form of contraception.

Exclusion Criteria

• Inability to give informed consent.
• Respiratory culture positive for B.cepacia complex
• Renal function abnormalities-Creatinine greater than 1.5 times upper limit of normal within a 30 day period prior to randomization
• Liver function abnormalities : AST or ALT greater or equal to 2.5 times the upper limit of normal within a 30 day period prior to randomization
• Neutropenia, absolute neutrophil count< or = 1000 within a 3-day period prior to randomization
• History of biliary cirrhosis documented by liver biopsy or imaging.
• History of portal hypertension.
• Investigational drug use within 30 days of randomization date.
• History of alcohol, illicit drug or medication abuse within 1 year of randomization.
• Women who are pregnant, breastfeeding or trying to conceive