To determine if the prevalence of P. aeruginosa resistance and the prevalence of other CF pathogens has changed over the past ten years, this multi-center cross-sectional study will enroll a contemporary cohort of patients with CF and will compare their sputum microbiology and susceptibility data with corresponding baseline data from an historical cohort consisting of patients enrolled in the prior phase 3 trials of inhaled tobramycin, conducted by PathoGenesis Corporation. (A Phase III Placebo Controlled Clinical Trial (PC-TNDS-002) to Study the Safety and Efficacy of Tobramycin in Inhalation in Patients with Cystic Fibrosis). The data set from the phase 3 trials which will be used for this research project will be anonymous.