Although pulmonary function tests are used to evaluate acute changes in obstructive airway disease in patients with cystic fibrosis (CF), these tests are relatively difficult to perform in young children or severely ill patients and may be costly. Other standard tests (eg, the Shwachman-Kulczycki and National Institutes of Health [NIH] scoring systems) evaluate disease severity and predict prognosis but do not measure day-to-day changes in clinical status. They thus provide little information for assessing the start of acute pulmonary exacerbation. Alternative scoring systems are needed to better identify the start of pulmonary exacerbation, to predict worsening or improvement of respiratory function after intervention, and to distinguish the scores from illness severity scores. This study was undertaken to compare a new 10-component, 50-point-maximum, acute clinical scoring system with forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) variables in children with CF who were experiencing an acute pulmonary exacerbation before antimicrobial therapy was initiated and until the end of therapy. One hundred thirty children aged 5 to 17 years (median age, 11 years) had a median NIH score of approximately 64 (range, 39 to 85) at admission. The cystic fibrosis clinical score (CFCS) at admission was found to correlate highly with the modified NIH score at study entry (r = -.68, P = 0.0001 ). The total CFCS at entry was correlated inversely with both FEV1 (r = -.57, P = 0.0001) and FVC (r = -.55, P = 0.0001) measurements; crackles, dyspnea, sputum production, and respiratory rate were the 4 components most highly associated with either pulmonary function variable. The change in total CFCS from start to end of antimicrobial therapy also correlated with changes in FEV1 (r = -.31, P = 0.0016) and change in FVC (r = -.47, P = 0.0001). Clinical improvement was observed in all patients at the end of therapy, and only 1 patient had an increase in total CFCS. Patients who experienced clinical relapse had a mean increase of 8.5 points in the CFCS from end of therapy to 2- to 4-week follow-up, indicating worsening signs and symptoms of acute exacerbation. These data suggest that the CFCS is a predictive and optional surrogate of pulmonary function in assessing the health of patients with CF. Following further validation, this scoring system could be used to evaluate health status in the outpatient setting, the need for hospitalization, and subsequent improvement during an acute pulmonary exacerbation, as well as to compare the efficacy of therapeutic regimens.