The plasma protein mannose-binding lectin (MBL) activates the complement system by binding to carbohydrate structures presented by microorganisms and thus could be an important component of the innate immune defence system. We measured MBL in patients with leukaemia who were scheduled to undergo chemotherapy (ie, a population especially susceptible to infection) and related the results to severity of infection after chemotherapy. We showed a significant association between low concentrations of MBL and serious infections related to chemotherapy (p