Sepsis is associated with abnormalities of muscle tissue oxygenation and of microvascular function. We investigated whether the technique of near-infrared spectroscopy can evaluate such abnormalities in critically ill patients and compared near-infrared spectroscopy-derived indices of critically ill patients with those of healthy volunteers. We studied 41 patients (mean age 58 +/- 22 years) and 15 healthy volunteers (mean age 49 +/- 13 years). Patients were classified into one of three groups: systemic inflammatory response syndrome (SIRS) (n = 21), severe sepsis (n = 8) and septic shock (n = 12). Near-infrared spectroscopy was used to continuously measure thenar muscle oxygen saturation before, during and after a three-minute occlusion of the brachial artery via pneumatic cuff. Oxygen saturation was significantly lower in patients with SIRS, severe sepsis or septic shock than in healthy volunteers. Oxygen consumption rate during stagnant ischaemia was significantly lower in patients with SIRS (23.9 +/- 7.7%/minute, P < 0.001), severe sepsis (16.9 +/- 3.4%/minute, P < 0.001) or septic shock (14.8 +/- 6%/minute, P < 0.001) than in healthy volunteers (35.5 +/- 10.6%/minute). Furthermore, oxygen consumption rate was significantly lower in patients with septic shock than patients with SIRS. Reperfusion rate was significantly lower in patients with SIRS (336 +/- 141%/minute, P < 0.001), severe sepsis (257 +/- 150%/minute, P < 0.001) or septic shock (146 +/- 101%/minute, P < 0.001) than in healthy volunteers (713 +/- 223%/minute) and significantly lower in the septic shock than in the SIRS group. Near-infrared spectroscopy can detect tissue oxygenation deficits and impaired microvascular reactivity in critically ill patients, as well as discriminate among groups with different disease severity.