OBJECTIVES:  To determine if the average cost of medical care among cystic fibrosis (CF) patients post Pseudomonas aeruginosa (PA) infection is different for those with underlying comorbid conditions.  METHODS:  MarketScan claims database which captures person-specific direct medical utilization, expenditures, and enrollment from approximately 150 payers was used to conduct a retrospective cross-sectional study.  CF subjects with an initial claim for a PA infection were identified using international classification of diseases diagnosis codes of 277.0 and 482.1, respectively. Demographic information from administrative claims and health care utilization and costs from medical and pharmacy claims were extracted for 12 months post initial PA claim.  All resource use and costs were annualized and compared among subjects with comorbidity and the overall sample using SAS version 9.2.  RESULTS: Of the 347 CF subjects with PA infection that met the study criteria, 79% had multiple co-morbidities, 67% had pancreatic insufficiency, followed by chronic sinusitis (19%), CF-related diabetes (13%), bronchiectasis (7%), and osteoporosis (5%). Subjects had a mean age of 20 years (SD 15.4) and 47.8% were females. As compared to an average CF subject, those with multiple co-morbidities had significantly higher (p<.05) overall pharmacy claims (mean 39 vs. 44) and cost (mean $20,673 vs. $24,121, median $17,121 vs. $22,344). Overall cost in subjects with CF-related diabetes (mean=$66,364 and median=$60,759) and osteoporosis (mean=$88,703 and median=$55,686) was significantly (p<0.05) greater as compared to an average CF subject (mean=$51,821 and median=$36,046). Among CF-related diabetic subjects and osteoporosis subjects, significantly (p<0.05) higher outpatient visits and pharmacy claims and higher inpatient costs contributed to overall increase in health care costs compared to the average CF subject.  CONCLUSIONS:  Overall post-PA infection medical care costs were higher in comorbid cohorts compared to the overall CF sample.  Future research needs to determine cost-effectiveness of PA infection treatment for among these comorbid conditions.