Mannose-binding lectin (MBL; also known as mannan-binding lectin) is an important component of innate immunity. MBL levels are mainly genetically determined. Low serum MBL levels and their cognate haplotypes have been associated with a wide range of infections. However, most subjects with MBL deficiency remain healthy. MBL deficiency is also associated with non-infectious diseases including systemic lupus erythematosus, rheumatoid arthritis, cystic fibrosis and common variable immunodeficiency. MBL deficiency may affect susceptibility to (e.g. meningococcal disease), or alter the natural history of (e.g. rheumatoid arthritis, cystic fibrosis), a disease. MBL (plasma-derived or recombinant) therapy has yet to be shown to be safe and effective. Potentially it may be useful in MBL-deficient patients to reduce susceptibility to, or enhance recovery from, bacterial infection or to alter the natural history of a disease (disease-modifying drug). In practise the place of MBL therapy may be as a disease-modifying drug to reduce the severity of rheumatoid arthritis and to preserve lung and liver function in cystic fibrosis. MBL therapy may also ameliorate various immunodeficiency syndromes. A potential hazard of MBL therapy is enhanced complement-mediated host damage. The place of MBL therapy will await results of randomized controlled clinical trials.