Presently, effectiveness of treatments for CF lung disease is judged by improvement in lung function (FEV1). However, in CF patients, FEV1 can range from severely decreased to normal, and improvements may occur slowly. Thus, clinical trials require many patients over prolonged periods to evaluate medications. As the pace of drug development accelerates, it is no longer possible to test all of the promising candidate therapies using conventional study designs. A sensitive technique for assessing lung inflammation has been developed which uses the expression of genes located in circulating blood cells. Mononuclear cells pass repeatedly through the blood vessels of the lung, and are exposed to many of the inflammatory products that are present in the airways. Over the past 4 years the investigators have identified a small group of candidate genes that are unregulated or downregulated in response to antibiotic treatment. The investigators now propose to prospectively test this method of quantifying lung inflammation in a large group of CF patients undergoing treatment of pulmonary exacerbations. Blood will be sampled before and after antibiotic treatment for a pulmonary exacerbation, and the relative change in gene expression will be compared to improvement in FEV1 and other clinical responses, to determine the utility of this method for use in studies. If successful, this technique could allow for a rapid and noninvasive method to gauge immediate effects by new treatments, and assist caregivers in determining optimal treatment strategies for the individual.