A randomised double blind 2-week trial of monotherapy with ceftazidime and aztreonam was completed in 22 adults with cystic fibrosis. 26 courses of treatment were initiated and 22 courses were completed. Within groups, both ceftazidime and aztreonam demonstrated a significant improvement in FEV1 at 2 weeks (ceftazidime p less than 0.05; aztreonam p less than 0.01) and a decline in sputum weight at 2 weeks (ceftazidime p less than 0.05; aztreonam p less than 0.05). Between groups, for spirometry and sputum weight the only change was a slower decline in FEV1 in the aztreonam group at 42 days (p less than 0.05).