ObjectiveTo determine the functionality of identified polymorphisms in the promoter and upstream regions of the interleukin-10 gene in terms of release of interleukin-10 from lipopolysaccharide-stimulated whole blood from healthy volunteers and to evaluate the relationship of interleukin-10 polymorphisms to interleukin-10 release, development of sepsis, and mortality in critically ill patients.DesignObservational study.SettingThe academic unit of anesthesia and intensive care, university laboratories, and ten-bed general intensive care unit in a university teaching hospital.SubjectsA total of 132 healthy volunteers plus 67 consecutive critically ill patients recruited within 24 hrs of admission to the intensive care unit, regardless of diagnosis.MeasurementsPlasma interleukin-10 levels were measured by enzyme-linked immunosorbent assay. Single nucleotide polymorphisms were detected by restriction fragment length polymorphism analysis. Dinucleotide repeat polymorphisms were identified after polymerase chain reaction using a DNA size analyzer.Main ResultsStimulated interleukin-10 release in critically ill patients was significantly lower than in healthy subjects (p< .0001). In addition, in the patients who developed sepsis, interleukin-10 release at admission to the intensive care unit was significantly lower than in patients who did not subsequently develop sepsis (median [range] 1.47 [0.13–6.90] ng/mL compared with 4.93 [0.03–16.80] ng/mL,p= .001). The A allele of the single nucleotide polymorphism at −592 base pairs was associated with lower interleukin-10 release and higher mortality in critically ill patients. Other polymorphisms were not linked to interleukin-10 release, sepsis, or mortality.ConclusionsThe A allele of the −592 base pair single nucleotide polymorphism in the interleukin-10 gene is associated with lower stimulated interleukin-10 release and increased mortality. Further investigations are required to determine the nature of the functionality and the potential diagnostic and therapeutic aspects of this marker.