Heme oxygenase 1 (HO-1) is a cytoprotective enzyme upregulated by various critical illness-related stress stimuli. We investigated the association of HO-1 gene polymorphisms and plasma concentrations with the outcome of critically ill patients in a prospective cohort study of 231 critically ill patients admitted to tertiary care medical and medical-surgical intensive care units. Blood samples were collected on days 1, 2, and 3 to 4 in the intensive care unit. The HO-1 plasma concentration was measured in serial samples, and HO-1 single nucleotide polymorphisms, -413A/T and +99G/C, and HO-1 promoter GTn repeat length polymorphism were determined. The +99C and long GTn alleles were in perfect linkage disequilibrium, and the -413T/GT(L)/+99C haplotype had significant independent effect on first-day HO-1 plasma concentrations in linear regression analysis (P = 0.03) and associated with lower HO-1 plasma levels. Furthermore, -413T/GT(L)/+99C haplotype associated with a lower frequency of multiple-organ dysfunction compared with other haplotypes (P = 0.017). The HO-1 plasma concentrations of study patients were significantly higher than the values of healthy controls at all time points (P < 0.001), and the first-day plasma HO-1 levels were independently associated with the Sequential Organ Failure Assessment score (P = 0.001). In conclusion, the HO-1 -413T/GT(L)/+99C haplotype is associated with HO-1 plasma levels and the frequency of multiple-organ dysfunction in critically ill patients. The HO-1 plasma concentrations are significantly increased among critically ill patients and associated with the degree of organ dysfunction.