• In vitro and animal experimental data suggest the combination of edetate sodium (EDTA) by aerosol plus oral antimicrobials might be effective in the treatment of chronic Pseudomonas infection in patients with cystic fibrosis (CF). For six months we studied the effects of edetate sodium administered by ultrasonic nebulizer to ten children with CF and chronic Pseudomonas aeruginosa infection in a double-blind, placebo-controlled, crossover study. The children had evidence of mild to moderate disease at entry in the study, with a mean (±SD) forced expiratory volume in the first second of 85% (±18%0 of the predicted value and a mean (± SD) Shwachman-Kulczycki score of 83 (±7)/100. Each child was on a three-month regimen of aerosolized edetate sodium plus oral tetracycline twice daily followed by three months of placebo aerosol plus tetracycline or vice versa. Progress was assessed by measurement of pulmonary function, physical examination, and sputum cultures at four weekly intervals, plus chest roentgenograms on entry and after each of the three-month treatment periods. Daily symptoms were assessed using a diary card system. Two patients could not complete the study, one because of severe respiratory relapse, the other because of antibiotic side effects. Of the remaining eight patients, none showed any improvement in pulmonary function, weight gain, or growth acceleration, and none was rendered free of Pseudomonas lung infection. Daily symptom scores and chest roentgenograms were unaltered by edetate sodium. We conclude that the combination of aerosol edetate sodium plus oral tetracycline over a three-month period does not modify the clinical course nor the pulmonary flora in patients with CF with chronic Pseudomonas lung infection.