Mannose-binding lectin (MBL) genetic polymorphisms result in deficiency of the encoded protein and increased susceptibility to infection, especially in children and the immunocompromised. The objective of this study was to investigate the relationship between MBL-2 exon 1 and promoter -221 polymorphisms, plasma levels of the encoded protein, and the incidence and outcome of severe sepsis and septic shock. One hundred seventy-four white adult patients with severe sepsis or septic shock were recruited in a prospective multicenter study across eight intensive care units in the South of England, UK. Genotype and haplotype frequencies were compared between normal population controls and patients, and between survivors and nonsurvivors. Plasma levels of encoded protein were related to genotype and outcome. The exon 1 polymorphisms (A/O or O/O) were significantly more common in the patients with severe sepsis and septic shock than in normal healthy adults (54.6% vs. 39.7%, P = 0.001), and there was a significant difference in haplotype frequency between controls and septic patients (P < 0.0001). There was no significant difference in MBL-2 genotype or haplotype frequency between survivors and nonsurvivors. There was a strong relationship between MBL-2 haplotype and plasma MBL concentration (P < 0.001). Individual plasma levels were variable and increased between days 1 and 7. The mortality rate was higher in those with MBL levels <1000 microg/L than in those patients with levels >1000 microg/L (47.2 vs. 22.2%, P = 0.05). We conclude that genetic polymorphisms resulting in mannose-binding lectin deficiency are associated with increased susceptibility to sepsis. The close relationship between polymorphic variants and plasma MBL concentration persists during sepsis but individual levels vary widely. Lower circulating MBL levels are associated with a poor outcome.