Severe sepsis is a systemic inflammatory response to infection resulting in acute organ dysfunction. Vascular perfusion abnormalities are implicated in the pathology of organ failure, but studies of microvascular function in human sepsis are limited. We hypothesized that impaired microvascular responses to reactive hyperemia lead to impaired oxygen delivery relative to the needs of tissue, and that these impairments would be associated with organ failure in sepsis. We studied 24 severe sepsis subjects 24 hours after recognition of organ dysfunction; 15 healthy subjects served as controls. Near-InfraRed Spectroscopy (NIRS) was used to measure tissue:1) microvascular hemoglobin signal strength; and 2) oxygen saturation of microvascular hemoglobin (S_tO₂). Both values were measured in thenar skeletal muscle before and after 5 minutes of forearm stagnant ischemia. At baseline, skeletal muscle microvascular hemoglobin was lower in septic than control subjects. Microvascular hemoglobin increased during reactive hyperemia in both groups, but less so in sepsis. S_tO₂ at baseline and throughout ischemia was similar between the two groups, however the rate of tissue oxygen consumption was significantly slower in septic subjects than in controls. The rate of increase in S_tO₂ during reactive hyperemia was significantly slower in septic subjects than in controls; this impairment was accentuated in those with more organ failure. We conclude that organ dysfunction in severe sepsis is associated with dysregulation of microvascular oxygen balance. NIRS measurements of skeletal muscle microvascular perfusion and reactivity may provide important information about sepsis, and serve as endpoints in future therapeutic interventions aimed at improving the microcirculation.