Mannose-binding lectin (MBL) plays a crucial part in activation of the lectin pathway of the complement system. Increased susceptibility to certain infections is associated with low serum levels of MBL due to MBL variant alleles, but the contribution of MBL insufficiency to incidence of common childhood infections is unknown. In the 14 March Journal of the American Medical Association Anders Koch and colleagues report evidence that MBL insufficiency increases the risk of acute respiratory tract infection (ARI) in children (JAMA 2001, 285:1316-1321).