Sputum samples from the lungs of cystic fibrosis patients harboring Pseudomonas aeruginosa infections were collected and examined for the presence of the siderophore pyoverdine. Fluorescence quenching, due to the addition of ferric ion, as well as column and thin-layer chromatography results indicated that all samples contained the siderophore. Six samples furnished sufficient material after purification to allow us to obtain visible absorbance spectra. These spectra were characteristic of the ferrated analog of the P. aeruginosa pyoverdine, that is, ferripyoverdine, and in all cases they indicated a degree of ferration in excess of 50%. P. aeruginosa in the cystic fibrosis lung is thus iron stressed and responds by synthesizing pyoverdine, which subsequently binds ferric ion.