Nine variables were studied in 56 patients to analyze hemodynamic patterns of critically ill and shock patients. The variables were central venous pressure, mean arterial pressure, heart rate, cardiac index, left ventricular stroke work, strok index, total peripheral resistance, arteriovenous oxygen difference, and oxygen consumption. We observed six patterns; three with low cardiac index (hypodynamic) and three with high cardiac index (hyperdynamic). Group IA: Low cardiac index with increased central venous pressure and arteriovenous oxygen differences associated with myocardial infarction, cardiac insufficiency, and postoperative cardiac surgery: Group IB: Low cardiac index with normal arteriovenous oxygen difference associated with myocardial infarction or hypovolemia. Group IC: Low cardiac index and decreased arteriovenous oxygen difference in patients with hypodynamic septic shock. Group IID: High cardiac index and increased arteriovenous oxygen difference in patients with sepsis and stable hemodynamic conditions. Groups IIE and IIF: Increased cardiac index and normal or increased arteriovenous oxygen difference in septic patients, who were hemodymamically unstable or in shock. These hemodynamic observations were found to be useful for understanding physiological compensations, for deciding on therapy, and in evaluating the effectiveness of therapy.