One molecule that is inhibited by doxycycline is matrix metalloprotease-9, which is emerging as an important mediator of lung inflammation and damage in cystic fibrosis. We hypothesize that the addition of treatment with doxycycline in CF inpatients will reduce MMP-9 activity and inflammatory markers in the sputum of cystic fibrosis patients compared to CF patients not treated with doxycycline.