BACKGROUND Patients with cystic fibrosis (CF) frequently experience pulmonary exacerbations that may lead to a faster subsequent decline in pulmonary function; however, this relationship has not been clearly established. The purpose of this study was to determine the association between the frequency of pulmonary exacerbations and subsequent forced expiratory volume in 1 sec (FEV(1) ) decline in adults and children with CF. METHODS Cohort study of subjects followed in the Cystic Fibrosis Foundation Patient Registry from 2003 through 2006. Mixed effects modeling was used to estimate differences in rates of decline in FEV(1) in 2004-2006 for patients with 0, 1, 2, or 3+ pulmonary exacerbation(s) in 2003. RESULTS Of 8,490 subjects who met inclusion criteria, 60% had 0 exacerbations, 23% had 1, 10% had 2, and 7% had 3+ exacerbations in 2003. Compared to children with no pulmonary exacerbations in 2003, children with one or more exacerbations experienced a significantly (P < 0.001) greater rate of FEV(1) decline in 2004-2006. In contrast, among adults, only those with 3+ exacerbations in 2003 had a significantly (P = 0.01) greater rate of FEV(1) decline in 2004-2006 than those with no exacerbations in 2003. CONCLUSIONS There is a strong association between the frequency of pulmonary exacerbations and subsequent decline in pulmonary function. In adults, having 3+ exacerbations, and among children, having any exacerbations is associated with a greater rate of decline in the ensuing 3 years. Improved prevention, identification, and treatment of pulmonary exacerbations are likely to have long-term benefits for patients with CF, especially children.