OBJECTIVES: To determine if there were significant differences in pulmonary status (as measured by forced expiratory flow in one second [FEV1]) between care sites and subsequently to characterize the associations between these outcomes and practice patterns, including antibiotic use. METHODS: The Epidemiologic Study of Cystic Fibrosis contains detailed information on clinical status and therapies collected at physician encounters on more than 20,000 CF patients. Comparisons were done separately for four different age groups (<6, 6-12,13-17,>18). For each age group, sites were included if 10 or more patients in the age group had been followed from Jan 1995 to Dec 1996. For each site and age group, lung health was defined as the median of FEV1 values for the patients at that site. For each age group, the median values at eligible centers were ranked in order from the highest to lowest value. The mean FEV1 for all patients in the 25% of centers reporting the highest median values was compared to the mean values for the patients in 25% of centers with the lowest ranked medians. RESULTS: Large differences were observed between the centers ranked in the upper 25% compared to the lower 25%. (6-12 years; 92.3% Vs 74.6%, p<0.001. 13-17 years; 84.6% Vs 64.0%, p<0.001. >18 years; 64.4% Vs 49.7% p<0.001) Sites that reported higher values for FEV1 monitored patients more frequently, reviewed spirometry more often and treated patients more aggressively as indicated by the frequency of antibiotic interventions, the use of intravenous antibiotics and the duration of therapy. CONCLUSIONS: Frequent monitoring and aggressive intervention is associated with a better outcome in cystic fibrosis patients.