We classified 41 patients in septic shock on the basis of cardiac index (CI) after volume expansion with plasma protein solution, in order to obtain adequate filling pressures. Five had decreased CI (less than 3.5 1/min per m2), 31 had moderately increased CI (3.5 - 7.0 1/min per m2) and 5 had extreme hyperdynamic shock with CI superior to 7.0 1/min per m2. Among the patients with increased CI, those with extreme hyperdynamic state (EHS) had lower total systemic and pulmonary arteriolar resistances (370 vs 658 and 52 vs 119 dynes X s X cm-5, respectively) and a higher stroke index (67 vs 46 ml/m2), in spite of similar right atrial pressures. In this latter group, blood lactate was higher (6.5 vs 2.1 mmol/l), acidosis was more severe and coagulation disorders more pronounced; all five patients maintained an extremely high CI until death, which supervened after a brief episode of sinus bradycardia. A similar clinical course was rarely observed in the remaining moderately hyperdynamic group, in which mortality rate was significantly lower (35%). Three of five patients with EHS (compared to 2 of 31 in the moderately hyperdynamic group) had liver cirrhosis, the fourth died of fulminant meningococcemia and the fifth had prolonged polymicrobial bacteremia before adequate treatment was begun. Thus, underlying liver disease or particularly severe and uncontrolled infection seems to predispose to EHS. It is concluded that septic shock with extremely high cardiac output and excessively low peripheral resistances represents a distinct subset with more severe metabolic and coagulation disorders, an unusual hemodynamic evolution and a particularly poor prognosis.