Idiopathic pulmonary fibrosis (IPF) is an interstitial lung disease characterized by destruction of normal epithelial structure, proliferation of fibroblasts, and deposition of connective-tissue matrix proteins. There are currently no effective therapies for IPF. Over the past two decades, preclinical studies of inhaled low dose carbon monoxide (CO) have shown that this biologically active diatomic gas possesses properties that would make it a viable novel therapy for IPF. CO therapy has been well tolerated in Phase I and Phase II human trials to date. This phase II study is designed to investigate whether IPF patients show evidence of decreased peripheral blood levels of matrix metalloproteinase-7 (MMP7) and stability of secondary indicators of disease progression after 3 months of inhaled therapy.