Septic shock is characterized as a distributive form of circulatory failure. We examined the relationship of changes in forearm arterial, venous, and microvascular tone to the severity of sepsis. The study population consisted of ten control patients, 15 patients with sepsis, and eight patients with sepsis and shock. Patients treated with inotropic, vasopressor, or vasodilator drugs were excluded from the study. Forearm venous capacity (MVC), forearm venous tone (F-VT), arterial blood flow (FBF), forearm arterial resistance (FAR), and hyperemic response (FBF-RH) were measured using air plethysmography. MVC was decreased and VT increased in septic and septic shock patients. FBF was decreased with modest increases in FAR in septic and septic shock patients. FBF-RH was significantly reduced in both septic and septic shock patients. Decreases in FVT and attenuation of the reactive response to hyperemia occurred early in sepsis in patients without clinical evidence of hypoperfusion. In our patients, progressive vasodilatation in skeletal muscle was not associated with severe sepsis. These changes appear to be proportional to the severity of sepsis and are most pronounced in patients with circulatory failure.