This proof of mechanism study aims to evaluate the safety, tolerability and pharmacodynamics of SB-656933 following 28 days of daily administration of 20 and 50 mg SB-656933 in patients with CF compared to placebo. The primary endpoints of the study will be the effect of SB-656933 on safety and tolerability (adverse events, vital signs, clinical laboratory assessments, 12-lead electrocardiograms, and urinalysis) following 28 days of dosing. Secondary endpoints will include levels of neutrophil elastase in induced sputum and other sputum markers of inflammation (e.g. myeloperoxidase, total protein), induced sputum cells (i.e. total sputum neutrophil counts, percent sputum neutrophils) and sputum microbiology. Other assessments will include lung function measurements (spirometry); serum and plasma markers of inflammation (e.g. fibrinogen, CC-16, CRP, MMP8, MMP9, SP-D, and CXCL-8), quality of life questionnaire, and population pharmacokinetics parameters of SB-656933