An association between mannan-binding lectin (MBL) status and severity of lung function impairment in cystic fibrosis (CF) has been found in several studies, but not in others. To explore the possible basis for discrepancies in the literature, we related both MBL and l-ficolin concentrations to lung function and examined the results in relation to the age of the patients. For patients under 15 years of age, those with MBL < 200 ng/ml had better lung function than those with MBL > 200 ng/ml [median forced expiratory volume in 1 s (FEV₁), 99%versus 83%; P = 0·05]. For patients over 15 years of age, those with MBL < 200 ng/ml had poorer lung function than those with MBL > 200 ng/ml (median FEV₁, 44%versus 55%; P = 0·1). Also, for the over 15-year-olds, the proportion of patients with FEV₁ values below the median was greater in the MBL-insufficient subgroup (P < 0·04). In other words, relative deficiency of MBL appears to accelerate the age-related decline in lung function in CF patients. No corresponding relationships could be found between l-ficolin concentration and lung function. These findings and interpretation lend support to the potential value of MBL replacement therapy in a small minority of cystic fibrosis patients.