Blood volumes measured by indicator dilution method in over 1500 instances of critically ill patients of various etiologies and at various times throughout their critical illness were compared with the values of concomitantly measured mean arterial pressure (MAP), CVP, pulmonary arterial wedge pressure (WP), Hct, and cardiac output. During resuscitation from hypovolemic shock, the patients' blood volumes and the monitored variables were significantly altered. However, there were poor correlations between the extent of blood volume changes and these variables during resuscitation as well as throughout the critical illness, irrespective of the etiologic type or stage of shock. With administration of a fluid load, blood volume and values of the commonly monitored variables improved appropriately, but the correlation coefficients, in general, were not good. The data suggest that the commonly monitored variables, in and of themselves, do not reflect adequately the blood volume status in critically ill patients.