Deficiency of mannose-binding lectin (MBL), a serum protein involved in killing and promoting phagocytosis of pathogens, is associated with respiratory infection and disease progression in a number of acute and chronic lung diseases, including cystic fibrosis (CF)- associated bronchiectasis. No such association has been studied in non-CF bronchiectasis (nCF-Br). One hundred and thirty-three adult patients with nCF-Br were studied. Serum MBL levels were measured and deficiency defined using two cut-off levels, i.e. MBL ≤100 ng/ml and ≤600 ng/ml. Parameters of severity included lung function impairment, annual exacerbation and hospital admission rates, breathlessness, and Pseudomonas aeruginosa and Haemophilus influenzae infection rates. The incidence of MBL deficiency using cut-off levels of 100 ng/ml and 600 ng/ml was 10% and 26% respectively, similar to rates seen in the general population. There was no significant difference in mean FEV(1)% predicted between MBL deficient and sufficient patients at both cut-off levels (≤100 ng/ml: 63.8% vs. 64.6%, P = 0.91; ≤ 600 ng/ml: 66.5% vs. 63.9%, P = 0.56). In addition, exacerbation/hospital admission rates, symptoms of breathlessness and isolation/colonisation rates with P. aeruginosa and H. influenzae were similar in both groups at both cut-off levels. In conclusion, MBL deficiency is not associated with markers of disease severity in patients with nCF-Br.