The efficacy of aztreonam was compared to that of standard therapy consisting of tobramycin and azlocillin in the treatment of acute pulmonary exacerbations of cystic fibrosis in a randomized, open trial. Fifteen patients were randomized to each treatment. Responses were assessed based on changes in pulmonary and clinical scores, white blood cell counts, pulmonary function tests and quantitative bacteriology of sputum which were performed before, every 5 to 7 days during and on the last day of therapy. Patients in both groups responded to therapy and there were no statistically significant differences in changes in the above indicators of response with therapy between the two groups (P greater than 0.05). The incidence of detection of Pseudomonas aeruginosa isolates resistant to all three study antibiotics increased with therapy. Side effects were limited to transient elevations of liver enzymes (both groups) and rash and fever in one patient treated with azlocillin. Aztreonam represents effective therapy for pulmonary exacerbations of cystic fibrosis associated with susceptible pathogens.