To determine the prevalence of a mannan binding lectin (MBL) deficiency in children with increased susceptibility to infections and to investigate whether other coexisting immunodeficiencies affecting opsonisation are needed to render MBL deficiency clinically significant.343 serum samples were collected from 266 children with repeated infections, a single episode of severe infection, or prolonged symptoms relating to infection. The concentrations of MBL, immunoglobulin G (IgG), M (IgM), A (IgA), and IgG subclasses (IgG1-4) were analysed.MBL deficiency was found in nine children (3.2%), seven of whom had repeated infections or a single episode of severe infection. In two, initial symptoms and signs suggestive of infection eventually led to the diagnosis of an autoimmune disease--Still's disease in one and pauciarticular juvenile rheumatoid arthritis in the other. Among the children with MBL deficiency and infections, concomitant IgG subclass deficiency was detected in five and a transient low level of one or two IgG subclasses in two. Prevalence of an IgG subclass deficiency in children with MBL deficiency was higher than in those without (56% and 22%, respectively).MBL deficiency alone is not an independent risk factor for infection but may be manifested in association with another humoral immunodeficiency affecting opsonisation. Among children with MBL deficiency, those with juvenile rheumatoid arthritis were overrepresented. This calls for further study.