Patients with cystic fibrosis (CF) suffer from chronic bacterial infection of the airways, and many patients are infected with multiresistant bacteria. Combination antibiotic susceptibility tests, or antibiotic synergy tests, are in vitro tests that have been developed to allow clinicians to choose combinations of antibiotics that should be more effective at killing, or inhibiting, multiresistant bacterial pathogens. Only one randomised controlled clinical trial has been performed to determine whether combination antibiotic susceptibility testing leads to an improved clinical outcome for patients with acute pulmonary exacerbations of CF. The results of this clinical trial were disappointing - treatment based on combination antibiotic susceptibility testing was no more effective than treatment based on conventional culture and sensitivity testing. The adoption of antibiotic synergy testing as routine practice for patients with CF would be costly and would not be justified as there is insufficient evidence to suggest that the routine use of these tests improves clinical outcomes.