BACKGROUND Susceptibility testing results are not predictive of clinical response to antibiotic therapy in chronic Pseudomonas aeruginosa infections in cystic fibrosis (CF). We assessed the impact of reducing the number of routine susceptibility tests performed on clinical outcome in these cases. METHODS In June 2006, we introduced a protocol whereby susceptibility tests of P. aeruginosa isolates obtained from respiratory samples of people with CF were limited to those taken at the commencement of antibiotic therapy, when there was evidence of clinical failure or routinely if not tested in the previous 3 months. At all other times, isolates were identified and reported as normal but P. aeruginosa isolates were not subjected to susceptibility testing. RESULTS Over a 6 month period, P. aeruginosa was isolated on at least one occasion from 193 patients attending the Adult Cystic Fibrosis Unit. In this period, we reduced the number of routine susceptibility tests by 56% (from a projected 2231 tests on 872 samples to an actual 972 tests on 427 samples). We assessed the response to courses of intravenous antibiotic treatment administered during the 6 month study period in 2006 and for courses administered in the same patients during the same calendar months in 2005. No significant differences in median change of FEV1, FVC, C-reactive protein (CRP), white cell count, weight or duration of intravenous antibiotics were observed. The projected savings of this intervention were 3500 euros in consumables and 170 h (costed at 6500 euros) of laboratory staff time per annum, a total annual saving of 10,000 euros (6500 pounds sterling). CONCLUSIONS For CF units sending regular, routine sputum samples, a reduction in the number of susceptibility tests performed in cases of chronic P. aeruginosa infection can be carried out without impacting on short-term clinical outcomes.