Thirteen patients with cystic fibrosis (aged 11 to 32 years) who were hospitalized for exacerbation and who had sputum cultures positive for Pseudomonas organisms were treated initially for 4 days with bronchodilators and physiotherapy followed by the addition of antibiotic (14 days, n = 8) or placebo (14 days, n = 4; 7 days, n = 1). Tc-99m DTPA aerosol scintigraphy was performed on the day before bronchodilators and physiotherapy, on the day before antibiotic or placebo, and on the day after completion of antibiotic or placebo therapy. Scintigrams were evaluated for change in the number of nonventilated segments and change in the number of bronchial deposits of aerosol. Sixty-nine percent of patients showed improvement after bronchodilators and physiotherapy alone. Sixty-two percent showed further improvement after antibiotic or placebo was added; this improvement was independent of whether antibiotic or placebo was administered (P greater than 0.1). These aerosol scintigraphy results failed to demonstrate that the effectiveness of bronchodilators and physiotherapy is enhanced by antibiotics in the treatment of cystic fibrosis exacerbations.