The lectin pathway of complement activation is used by a collectin, mannan-binding lectin (MBL), and two ficolins, L-ficolin and H-ficolin, to opsonize microorganisms for phagocytosis. We published evidence recently that MBL insufficiency is associated with recurrent respiratory infections in childhood. We have now measured serum L-ficolin in 313 respiratory infection patients and 74 healthy control children. L-ficolin concentrations below the lower limit of the control group were found in 6% of the patients (P < 0·02) and were associated most strongly with children having co-existing atopic disorders (11%; P = 0·002). We suggest that L-ficolin may have a role in protection from microorganisms complicating allergic disease.